quadrajet service manual 1981

5/28/2018 Quadrajet Service Manual 1981

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:A A L Carburetor 9D- 5March 198 1Supersedes 9D-5Dated May 1973Models 4MV-4MC-M4ME-M4M CServ ice Manual

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IMPORTANT SAFETY NOTIC EProper service and repair is important to the safe, reliable operation of all motor vehicles .The procedures recommended by Rochester Products Division of General Motors an ddescribed in this service manual are effective methods of performing service operations .Some of these service operations require the use of tools specially designed for the purpose .The special tools should be used when and as recommended .It is important to note that this manual contains various CAUTIONS and NOTICES whic hshould be carefully read in order to minimize the risk of personal injury to service personne lor the possibility that improper service methods will be followed which may damage th evehicle or render it unsafe . It is also important to understand that these Cautions an dNotices are not exhaustive. Rochester Products could not possibly know, evaluate an dadvise the service trade of all conceivable ways in which service might be done or of th epossible hazardous consequences of each way. Consequently, Rochester Products has notundertaken any such broad evaluation . Accordingly, anyone who uses a service procedur eor tool which is not recommended by Rochester Products must first satisfy himsel fthoroughly that neither his safety nor vehicle safety will be jeopardized by the servic emethod he selects .

All information, illustrations, and specifications contained in this manual are based on th elatest product information available at time of publication approval. The right is reserved t omake changes at any time without notice .

CONTENT SGENERAL DESCRIPTIONOPERATING SYSTEMSMAJOR SERVICE OPERATIONS6

NOTE: USE THIS MANUAL PLUS SECTIONS 9D-5A & 9D-5S FOR COMPLETE SERVIC EINSTRUCTIONS .

1981 GENERAL MOTORS CORPORATIO NPRINTED IN U .S .A.


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TYPICAL MODELS-4MV-M4M E

Figure 1 Model 4MV Figure 2 Model M4M EGENERAL DESCRIPTIONThe Rochester Quadrajet is a four barrel, two stag ecarburetor of down draft design . It has been proven tobe dependable and a versatile performer as witnessed by

its broad application and usage since its first introduc-tion in 1965. Modern demands for greater fuel economyand improved emission control have resulted in signifi-cant refinement of the original design . The RochesterQuadrajet has evolved to the point of being an efficien tand sophisticated fuel control device that is right fo rthe times and yet maintains the serviceability that is s oimportant to those responsible for vehicle performanceand customer satisfaction .

The Quadrajet carburetor has two distinct and sepa-rate design stages. Each stage operates to provide ablend of economical operation and responsive engin eperformance .

The primary side of the carburetor (fuel inlet side )has two small bores. Each bore contains a triple ven-turi arrangement equipped with plain tube nozzles . Thetriple venturi provides excellent fuel atomization an ddelivery in the off idle and part throttle ranges of engin eoperation .

Fuel metering is controlled through the usage oftapered primary metering rods that are suspended withinfinely machined metering jets . The metering rods aremoved vertically by engine vacuum control . This is accom -plished by the use of a spring-loaded piston and rodhanger assembly and results in the control of fuel allowedto pass through the metering jets and to the engin eduring low speed and moderate speed operation .

Some models use multiple stage enrichment method sthat may incorporate additional rods, jets and/or powerpiston arrangements. There are models that have device sthat respond to changes in atmospheric pressure as well .The objective is to provide a high degree of sensitivit yto air/fuel mixture control at lower and midrange engin e

has the design features to meet these needs . The indi-vidual systems are described in detail later in sections o fthis text .

The secondary side of the Quadrajet has two largerbores. This increased area, when combined with that o fthe primary side of the carburetor, provides an air de -livery capacity that can satisfy a broad range of engin eoperating conditions .

An air valve, positioned above the secondary bores ,controls the fuel delivery for high demand conditions .This secondary air valve is connected mechanically to apair of secondary metering rods that are tapered and ar esuspended within a pair of fuel metering orifices . Thefuel passing through these secondary orifices is directedto a set of delivery nozzles located at the top of each secon -dary bore and below the air valve . The fuel flowthrough the nozzles is thereby controlled in direct pro -portion to air flowing through the secondary bores .

There are two basic carburetor model designs :First released in 1965, the 4MV is an automatic chok e

model designed for use with a manifold mounted thermo-static choke coil . The 4MC model also is an automaticchoke carburetor but with the choke thermostatic coi llocated in a choke housing mounted on the side of th efloat bowl. Except for choke systems, all models hav ebasically the same principles of operation (Figure 1) .

First released in 1975, the M4ME models differ fromthe conventional 4MV-4MC carburetor models in tha tthe M prefix designates modified , indicating th eprimary side is revised to accomodate an adjustabl emetering rod assembly with filler spool, or aneroid -metering rod assembly, where required, and the auxiliarypower piston assembly (if used) . The aneroid feature isnormally used to achieve altitude compensation in air /fuel mixtures (designated Models M4MCA-M4MEA )


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Later modified models use separated main well swith an aneroid cavity insert in the float bowl replacingthe adjustable metering rod or aneroid-metering rodassembly .On all models, the fuel chamber is centrally locate dto avoid problems of fuel spillage causing engine tur ncut-out and delayed fuel flow to the carburetor bores .The fuel chamber is relatively small in volume to reduc efuel evaporation during hot engine shutdown . A plasticbowl insert is included to reduce the free volume abov ethe fuel level to prevent fuel loss to the main meteringjets during critical vehicle maneuvers .

The float system has a single pontoon float and fue linlet valve for simplification and ease of servicing. Anintegral fuel filter (or strainer), and check valve (if used) located in the float bowl ahead of the fuel inlet valve, iseasily removed for cleaning or replacement .

All models have an Adjustable Part Throttle (A .P .T. )screw located either in the throttle body (since 1968 o n4MV-4MC models) or in the float bowl ( M modifiedmodels) to aid in refinement of fuel mixtures for goo demission control .

The thermostatic choke coil assembly on 4MV model sis heated by hot exhaust gases that flow through a specia lpassage in the intake manifold . On 4MC-M4MC-an dM4MCA models, the thermostatic coil assembly i swarmed by exhaust heated air supplied through a tub eto the choke housing mounted on the float bowl . M4MEand M4MEA models use an electrically heated thermo-static coil assembly to control choke mixtures afte rengine start and cold driveaway .

The throttle body is aluminum to reduce overall weightand to improve heat distribution, plus there is a thickthrottle body to bowl insulator gasket to keep excessiveheat from the float bowl causing fuel percolation .

Some Quadrajet applications use a shim between th ethrottle body and flange gasket . The shim is used t oprotect the carburetor aluminum throttle body from ex-haust gases flowing through the heat cross-over orexhaust gas recirculation passage in the intake manifold .

CARBURETOR IDENTIFICATIO NEarly Quadrajet carburetors (Figure 3) have themodel number identification stamped on a circular ta g

pressed into a recess on the float bowl casting in a fla tarea on the throttle lever side. Later model Quadrajet

Figure 3 Carburetor Identification Tag - E arl yModels

carburetors have no tag but include the part numberstamped vertically on the left rear corner of the floa tbowl casting adjacent to the secondary pick-up leve r(Figure 4). If replacing the float bowl, follow the manu-facturer s instructions contained in the service packag eso that the identification number can be transferred t othe new float bowl . Refer to the part number on th ebowl when servicing the carburetor .

Figure 4 Carburetor Identification Location -Late Mode l s

SERVICE FEATURESThe primary side of the carburetor has six operatin gsystems . They are float, idle, main metering, power ,pump, and choke. All systems receive fuel from one fue l

chamber .The following text covers the operating systems fo rease in trouble-shooting and also recommended serviceprocedures. There are some design variations betwee ndifferent models which will be covered in that part ofthe text pertaining to that particular system or serviceprocedure .

OPERATING SYSTEMSF LOA T S YS TE M

The Quadrajet carburetor has a centrally located fuelchamber in the float bowl (Figure 5) . The fuel chamberis centered between the carburetor bores to assure a nadequate fuel supply to all metering systems with respec tto car inclination or severity of turns .

The float system (Figures 6 or 7) consists of a fuelchamber, fuel filter (with check valve on M4MC-M4M Eapplications) or a fuel inlet strainer (some 4MV appli-cations), a single closed-cell plastic float pontoon wit hintegral float lever, a float hinge pin, float valve an dpull-clip, float valve seat (with or without windows ) ,and internal vents. Also, some applications include eitheran idle vent valve, vacuum vent switch valve, or external

C A R B U R E T O R CAR BU R ETO RM O D E L N O

C H A N GELETTER A AT( I5/

MO B Y RBUILT (JAN 1967 )MODELS 0,2G . 4 G MODELS M 4M

(EARLY )

NOPEGOIO NS T A M P

SHIF TDENT

MODEL NO

CARBURETOR IDENTIFICATIO N 4M T Y P E

ASSEM PLANT COD E

YEARDAY OF YEAR


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Figure 5 Centralized Fuel Chambe r

Figure 6 Float System 4MV-4M C

vent connector tube leading to the vapor canister, tovent excessive fuel vapors that form in the fuel chambe rabove the fuel surface that could disrupt hot engin eoperation . A plastic filler block is located in the top o fthe fuel chamber over the float valve to prevent fue lslosh in this area. On M4MC and M4ME models, eithera filler spool or an aneroid cavity insert is used in thefuel chamber to reduce fuel slosh on turns . Also, ametal baffle is added to the pump well fill slot in th efloat bowl of M4MC-M4ME models to reduce fuel slos hduring various vehicle maneuvers .

On most models, an integral 1-inch or 2-inch pleate dpaper fuel inlet filter, dependent upon model, is mountedin the front of the float bowl behind the fuel inlet nutto filter impurities from the incoming fuel. If used, acheck valve is pressed into the neck of the fuel filter .The check valve consists of a plastic disc contained in aplastic retainer. It is held in the normally closed positionby a small spring which exerts pressure on the checkvalve. When the engine starts and fuel flow pressure fro mthe fuel pump enters the inlet nut, it pushes the smal lcheck valve off its seat . Fuel flows past the valve int othe inside of the filter and continues on through the filte rto the float valve and seat . With the engine off, thecheck valve closes and shuts off fuel flow to the carbu-retor to prevent fuel leaks if a vehicle roll-over shouldoccur .The check valve retainer also has a flanged nec kwhich seals between the filter and fuel inlet nut .

NOTICE : If used, the check valve must b einstalled to meet government safety standard sfor roll-over . New service replacement filtersinclude the check valve where required .

The fuel filter is held in position by the force of aspring located between the filter assembly and the fue linlet nut cavity .

NOTICE : It is very important that the filterbe serviced according to recommended main-tenance intervals to prevent dirt and othe rimpurities from entering the carbureto rmetering orifices .

The float system operates in the following manner :Fuel flow from the fuel pump enters the carburetor

fuel inlet nut . It opens the check valve (if used) in thefilter against spring force and flows through the filterelement (or strainer), and then passes from the filterchamber up through the float valve seat and flows pastthe float valve on into the fuel chamber . As the in -coming fuel enters the fuel chamber, the float pontoonrises and forces the float valve closed, shutting off fue lflow at a prescribed level . As fuel is used from the fuelchamber, the float pontoon drops to open the float valveallowing fuel to again enter the fuel chamber . Thiscycle continues, maintaining a near constant fuel leve lin the fuel chamber for all ranges of engine operation .

EXTERNA LIDLE VEN TVALV E

FUEL INLE TFILTER-J

'CHECKCHECK VALVE VALVES EA T FLOAT SYSTEM


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9D- 5March 1981Page 6 IFDelcoover the edge of the float lever at the center as shownin Figures 6 or 7 . Its purpose is to assist in lifting thefloat valve off its seat whenever fuel level in the fuelchamber is low.

NOTICE: Do not place pull clip throug hsmall holes in top of float lever . Severe flood -ing will result.There are two types of float valves used in the Quadra-jet carburetor: One type is diaphragm assisted and th eother is the conventional needle and brass seat .The diaphragm assisted float valve (shown in the insetFigure 6) is used primarily with a smaller float and onengines equipped with high pressure fuel pumps . Thevalve seat is a brass insert and is pressed into the bow l

fuel inlet channel below the diaphragm needle tip . Theseat is not removable, as the valve tip is of a materia lwhich makes seat wear negligible . Care should be use dduring servicing so that the seat is not nicked, scored ,or moved. The float valve is factory staked and testedand should not be re-staked in the field .Fuel flow through the diaphragm assisted float valvevaries from the conventional float needle . With the con-ventional type (as shown in Figure 7) fuel flows fro mthe inlet filter and inlet channel up through the needleseat orifice past the float needle valve and spills overinto the float bowl. With the diaphragm type float valve fuel from the inlet filter enters the channel above th efloat valve tip. When fuel level is low in the bowl, th efloat valve is off its seat and fuel flows down past th evalve tip into a fuel channel which leads upward throughthe bowl casting to a point above normal liquid leve land spills over into the float bowl .The diaphragm type float valve differs in operationfrom the conventional float needle in that a larger sea torifice can be used to provide greater fuel flow to thefloat chamber and yet allow the use of a small float .

This is accomplished through a balance of forces actin gon the float valve and diaphragm against fuel pum ppressure. Fuel pressure entering the float valve chambertends to force the valve closed . However, the samepressure is also acting on the float valve diaphragm . Thediaphragm has a slightly larger area than the floatvalve head, therefore the greater pressure acting on th ediaphragm tends to push the valve off its seat . Theforce of the float arm acting on the valve stem, as thefloat bowl fills, overcomes this pressure difference andcloses the valve . Therefore, the float's function is to over-come the pressure difference and it does not have t oforce a valve closed against direct fuel pump pressur eas does the conventional needle type .

To improve hot engine starting and to prevent roughidle which may'result from excessive fuel vapors tha tmay form in the fuel chamber, various methods havebeen used to vent these vapors external to the carburetor .Early Quadrajet applications vented these vapors t o

the atmosphere through an idle vent valve that is operated

mechanically. A wire tang on the pump lever opensthe valve during idle and allows the valve to close a tgreater throttle valve angles as in the off idle and par tthrottle positions .Some models use a thermostatically controlled ven tvalve (Figure 8) . This heat sensitive valve is operated bya bi-metal strip which holds the valve closed at tempera-tures below 75F. When temperatures at the carburetorexceed that value the valve opens allowing vapors toescape. The thermostatic vent valve is adjustable t omake sure it is timed to open during idle and closed athigher engine speeds .

Figure 8 Thermostatic Idle Vent Valv e

Figure 9 Bowl Vent ValveSome Quadrajets have a vacuum operated vent valv e

(Figure 9) that is also controlled to function with a nevaporative vapor canister . The vacuum vent valve i sdesigned to be open and to allow fuel chamber vapor sto be routed directly to a vapor collection canister in -stead of the atmosphere for improved evaporativeemission control during engine shut-down . A passagebeneath the vacuum diaphragm in the air horn pro -

VACUUM PASSAGEVACUUM VEN T VALVE

VALV ECONSTANT BLEEDR E ST R I C T I O N

VENT TOCANISTER


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B O W L V E N T V A LV E T O C A N I S T E R

FIXEDIDLEAIRBY-PASS

DLE CHANNE LRESTRICTION

E MIXTURE \HROTTLVALV ENEEDLE IDL HODISCHARGE HOLE COMPENSATORLVEIDLE

VAT

FUEL

OFF-IDLEOPERATION IDLE S YSTE MFigure 10 Bowl Vent Valv e

vides a constant purge of the vapors from the vapo rcanister during off idle and higher engine spee ds .Later modified versions of the Quadrajet have abowl vent valve that is spring-loaded and is actuatedby action of a spacer on the pump plunger shaft (Figur e10) . The fuel c hamber is externally ve nted to the vapo rcollec tion canister during periods of engine shut-dow n .Anothe r me thod used to vent vapors is by the additio n

of a tube to the air horn (see Figure 7) . Vapors flowthrough this tube and connecting hose to a vacuu moperated vapor vent valve that is an integral part of th evapor collection canister located elsew here in the engin ecompartment . The canister vent valve is spring-loade dand normally open, al low ing bowl vapors to pass into th ecanister during engine shut down. Manifold vacuumduring engine operation closes the valve and vapors ar epurged from the canister . This system improves hotengine starting and also mee ts government e vaporativ eemission re quirements .

NO TI CE : External venting of fuel chambe rvapors is not 'used on Q uadrajet carburetor sdesigned for marine use .

IDLE SYSTEMThe Quadrajet carburetor idle system is located o nthe primary side (fuel inlet side) of the carburetor t osupply the correct air/fuel mixture during idle and off-idle operation .The idle system is used during this period becauseair flow through the carburetor venturi is not grea tenough to obtain efficient metering from the main dis-charge nozzles . The idle system is only used in the tw oprimary bores of the c arburetor . Each bore has a separat eand independent idle circuit (Figure 11 ) .The idle system consists of a calibrated idle tube , ai rbleeds, idle channel restriction, idle mixture dischargehole, and idle mixture screws or needles (one for eac h

Figure 11 Idle System - Typica l

The idle system op erates as follow s :During curb idle (w arm engine), the primary throttl evalve is held slightly open by the idle speed screw orsolenoid plunger . The small amount of air passing be-tween the throttle valve and bore is regulated by ad-justing the position of the spee d scre w or solenoid plun-ger to obtain the desired idle speed. Since the enginerequires very little air for idle and low speed, fuel i sadded to the air to produce a combustible mixture b y

the direct application of vacuum (low pressure) fro mthe intake manifold to the idle discharge hole be low th ethrottle valve . The idle discharge hole is in a very lowpressure area and the fuel in the fuel chamberis vented to atmosphere (high pressure through the ai rcleaner) . This causes fuel to flow from the fuel chambe rdown through the main metering jet into the main fue lwe l l . It is picked up in the main well by the idle tube s(one for each bore) w hich extend into the fuel w el l . Thefuel is metered at the lower tip of the submerged idl etube and passes up through the tube. At this point, thefuel is mixed w ith air at the top of each idle tube throug hthe top idle air bleed. The air bleed size is controlledby either a drilled hole or a brass insert dependin gupon carbure tor application .

NO TI CE : No attempt should be made toinstall a brass insert in those app lication sthat use a drilled hole for the top idle ai rbleed Then the fuel mixture crosses over to the idle downchannel w here it is mixed w ith air at the side idle blee dlocated just above the idle channel restriction . The mix-ture continues dow n through the ca librated idle c hanne l

restriction past the lower idle air bleed and off-idl edischarge port where it is further mixed with air . Theair/fuel mixture moves down to the adjustable mixturescrew discharge hole w here i t enters the carburetor bor eand blends with the air passing by the slightly ope nthrottle valve . The combustible air/fuel mixture thenpasses through the intake manifold to the engine


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The idle mixture screws are adjustable to blend thecorre ct amount of air/fuel mixture from the idle syste mto the engine at idle . Turning the mixture screw s inw ar d(clockwise) decreases idle mixture discharge (gives al eaner m ixture) and turning the mixture scre w s outwar d(counter-clockwise) increases idle mixture discharge(gives a richer m ixture) .

OFF-IDLE OPERATIONAs the primary throttle valve is opened from cur bidle to increase engine speed, additional fuel is neededto combine with the extra air entering the engine . Thi sis accomplished by the slotted off-idle discharge port .As the primary throttle valve opens, it passes by th e

slotted off-idle discharge port, gradually exposing it tohigh manifold vacuum . The mixture added from the off -idle port mixes with the increasing air flow past th eopening throttle valve to maintain the required air/fue lmixture of the engine .Further opening of the throttle valve increase s the ai rvelocity through the carburetor venturi sufficiently t ocause low pressure at the lower idle air bleed. As aresult, fuel begins to discharge from the lower idle ai rbleed hole and continues throughout operation of th epart throttle to w ide-open ranges, thereby supplementin gmain discharge nozzle delivery .The idle mixture screw discharge hole and off idl edischarge port continue to supply sufficient fuel fo rengine requirements until air velocity is high enoughin the ve nturi area to obtain efficient fuel flow from themain meter ing system .The idle system functions in a similar manne r in eac hcarburetor bore .The secondary throttle valves remain closed duringidle conditions and on some Quadrajet applications the yare coated with a special graphite material which isapplied at the factory to effectively seal the secondary

throttle valves in the bores for minimum air flow a tidle to prevent possible rough idle .On m any exhaust emission control carburetor applica-tions, the idle mixture screw discharge holes have bee nreduced in size . This was done to prevent a too richidle adjustment in the field should the idle mixture sc rew sbe turned out too far beyond normal idle mixture re-quirements . Also, starting in 1971, idle screw limite rcaps w er e added to emission control carbure tors to dis-courage adjustment of the scre w s in the field . On latermodels, the idle mixture screws are sealed to preventreadjustment from the factory se tting in the field .

ADJUSTABLE AIR BLEE DAnother feature added to some emission carburetorsis an adjustable off-idle air blee d system (Figure 1 1) . Aseparate air channel is added in the air horn w hich leads

Figure 12 Idle System - Late Model sfrom the top of the air horn to the idle mixture crosschannel . An adjustment screw with a tapered head i smounted at the top of the channel and is used to con-trol the amount of air bleeding into the idle system .The off-idle air ble ed is adjusted at the fac tory to main-tain very accurate off-idle air/fuel mixture ratios . It isadjusted during carburetor flow test and no attemptshould be made to readjust in the field. A triangularspring clamp force d over the vent tube covers the scre wto protect the adjustment from be ing tampere d w ith an dit should not be r emoved . Al l service air horns have thisscrew preset at the factory .

FIXED IDLE AIR BY-PASSA fixed idle air by-pass system is used on some ap-plications which consists of air channels that lea dfrom the top of each carburetor bore in the air hor n

to a point below each throttle valve . At normal idle extra air passes through these channels supplementin gthe air passing by the slightly open throttle valves . Thepurpose of the idle air by-pass system is to reduce th eamount of air going past the throttle valve s so that the yare nearly closed at idle . This reduces the amount ofair flowing through the carburetor venturi to preven tthe main discharge nozzles from feeding fuel duringidle oper ation . The triple venturi system is very sensi-tive to air flow and where large amounts of idle airare needed to maintain idle speed, the fixed idle airby-pass system is used .PURGE PORTIN G

To meet evaporative emission standards on late mode lQuadrajet applications, fuel vapors from the carbu-retor float bowl and fuel tank are collected in a vapo rcanister and not vented to atmosphere . O n many of these

CANISTER THROTTLEPURGE TUBE VALVE

IDLE IDLE DISCHARG EMIXTURE OFFIDLE HOL E-NEEDL E

E .G .R . (EXHAUST GAS CANISTER PURG ERECIRCULATION)

IDLE SYSTEMOFF-IDLE

OPERATIO N

PORT CONSTANT


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carburetor models, vacuum ports are located in thethrottle body for canister purge. The purge ports leadthrough passages to a tube in the throttle body whic hconnects by a hose to the vapor canister .

The purge ports may consist of a constant (fixed )canister purge and a separate timed canister purge, ora timed canister purge only (Figure 12) .

The purge system operates as follow s :

CONSTANT CANISTER PURG EThe c onstant canister purge port ope rates during idl eoperation of the e ngine . It is located below the throttle

valves to provide a constant purge of the c anister w hen -ever the engine is running . As the throttle valves areopened beyond the idle position, additional purge of th ecanister is provided by eac h of the timed purge ports .TIMED CANISTER PURG E

The timed canister purge ports operate during off-idle part thrott le, and w ide-open throttle operation . They arelocated above the throttle valve in each bore next to th eoff-idle discharge port . The timed purge ports supplemen tthe constant canister purge port (w here used) to provide alarger purge capacity for the vapor canister and to pre -vent over-r ich mixtures from be ing added to the c arbu-retor m eter ing at any time .

On some applications (see Figure 12), the constantpurge port is used as the vacuum source to the EarlyFuel Evaporation (E.F.E . ) Valve loc ated in the exhaus tmanifold . On these models, purging of the canister i sacc omplished through the Positive Crankcase V entilation(P .C .V .) system .EXHAUST GAS RECIRCULATIO N

An Exhaust Gas Rec irculation (E .G .R .) system, con-sisting of an E.G.R. Valve, separate back-pressuretransducer valve (where used), vacuum hoses, an dvacuum supply ports in the Quadrajet carburetor, i sused on many vehicle applications to meet exhaustemission requirements . The E.G.R . valve is operatedby a vacuum signal taken from the carburetor . Thevacuum signal, dependent upon application, is takenfrom one or two punched ports located in the carbu-retor bore just above the throttle valves . Thus, E.G .R .valve ope ration is timed for metering exhaust gasesto the intake manifold dependent upon location of theports in the carburetor primary bore and by the degreeof throttle valve opening (See Figure 1 2) .It is important that the E .G .R . vacuum signal port(s )not be exposed to manifold vacuum during engin eidle and deceleration to keep the E .G .R . valve closed .This prevents rough idle which can be caused by ex-cessive exhaust gas contamination in the air/fuel mix-

TRANSMISSION CONVERTER CLUTC HSome applications have a port located above th ethrottle valve that is used to supply a timed vacuumsource for the automatic transmission converter clutch(see car division service manual for description of thi stransmission feature . )

HOT IDLE COMPENSATO RThe Hot Idle Compe nsator, w hen used on 4MV-4M C -M4M C Quadrajet carburetors (See Figure 11 ), is locate din a chamber at the rear of the carburetor float bow ladjacent to the sec ondary bores . Its purpose is to offset

the enriching effects caused by excessive fuel vapor sduring hot engine ope ration.The c ompensator consists of a thermostatically-c on-trol led valve , a he at sensitive bi-metal strip, and a valv eholder and bracket . The compensator valve assembly isheld in place by a dust cover over the valve chamber .A seal is used between the compensator valve and thefloat bow l casting . The valve closes off an air channelleading from a hole in the top of the air horn, jus tbeneath the air cleaner, to a point below the secondar ythrottle valves .Normally, the compensator valve is held closed b ytension of the bi-me tal strip. During e xtreme hot engin e

operation, excessive fuel vapors entering the enginemanifold cause ric her than normally required mixtures resulting in rough engine idle and stalling. At a pre -determined temperature, when extra air is needed tooffset the enriching effects of these fuel vapors, th ebi-metal strip bends and unseats the c ompensator valve .This uncovers the air channel leading from the valvechamber to the point below the throttle valves . Thisallow s enough air to be draw n into the engine manifol dto offset the richer mixtures and maintain a smoot hengine idle . When the engine cools and the extra air i snot nee ded, the bi-metal strip relaxes, cl oses the valve and operation returns to normal mixtures .For proper idle adjustment when the engine is hot the compensator valve must be c losed . To chec k this , afinger may be held over the compensator air inletchannel located on top of the air horn . If no drop inengine RPM is noted on a tachometer, the valve i sclosed . If the valve is open, plug the hole or c ool enginedow n to a point whe re the valve automatically closes fo rproper idle adjustment .

NO TI CE : Plug the comp ensator hole with apencil or something that will be seen, asthe plug must be removed before the aircleaner is installed . Otherwise the compen-sato r will not function if the plug is left i nthe hole .On some applications, the air inlet to the ho tidle compensator is located beneath the airvalve in the secondary bores . The air inlet in


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feeds additional fuel at closed choke for good cold enginestarting . Calibrated air bleeds, located in the air horn ,are used with this system .

The air-velocity sensitive pull-over enrichment systemallows the use of slightly leaner mixtures during par tthrottle operation and still provides enough fuel durin ghigh speed operation . This feature gives added refine-ment to the fuel mixtures for exhaust emission con-trol .

MAIN WELL AIR BLEED TUBE SSome 4MV Quadrajet applications use Main Wel lAir Bleed Tubes in place of the main well air blee d

holes (Figure 16) . The tubes, extending further into eac hprimary bore, help main discharge nozzle response a sthe main metering system starts to feed fuel .

MAIN METERING SYSTEM M4MC-M4M EMODELSStarting in the 1975 model year, major casting changeswere made to the air horn, float bowl, and throttl ebody of the Quadrajet carburetor to incorporate a ne w

Adjustable Part Throttle (A .P .T.) feature and, on som eapplications, the addition of an auxiliary power pistonand metering rod assembly. These new models weretermed modified and designated by the prefix Mto the model listing (e .g ., M4MC) .

ADJUSTABLE PART THROTTLEThe A.P.T. feature on these modified models con-sists of an adjustable metering rod assembly with fille rspool (Figure 17-A) or combination aneroid - meterin g

rod assembly (for an explanation of aneroid, see AltitudeCompensation, below) . The adjustable metering rod ,with or without aneroid, provides close tolerance controlof fuel flow to the main metering system during th epart throttle range .

The A.P.T. adjustment is performed at the factoryduring flow test by turning the threaded metering rod ,or aneroid-metering rod assembly, up or down to positionthe metering rod in a fixed metering jet located at th ebottom of the fuel reservoir in the float bowl . This set sthe part throttle air/fuel mixture to the desired flow band .

ALTITUDE COMPENSATIO NOn M4MCA-M4MEA models, a barometric pressure -

sensitive aneroid, sometimes called a bellows , is in-cluded as an integral part of the threaded A .P.T.metering rod assembly (Figure 17-B) . The aneroid, beingsensitive to air pressure change, automatically eitherexpands or contracts with changes of altitude to lowe ror raise the metering rod in the fixed metering jet tomaintain control of part throttle air/fuel ratios .

Figure 17 Adjustable Part Throttle - Modifie dModel s

Figure 18 Separated Main Well sN O T I C E : The position of the A .P.T. meter-ing rod in the fixed jet is extremely critical .Adjustment should NEVER be attemptedunless a replacement is required . The threadedA .P.T. metering rod should be adjuste dcarefully following adjustment procedures (Seestep 7a, page 38) .

As a result of these changes, a new float assembly ,plastic filler block (located in the top of the float cham-ber over the float valve), and new air horn gasket ar eused with the modified carburetor models .SEPARATED MAIN WELLS

The main metering system of most later modified Quadrajet models (Figure 18) operates similarly to tha tdescribed for 4MV-4MC models, above, except the floatbowl casting is revised to provide for separated mainwe l l s . The separated main well feature is used to feedeach main nozzle for improved fuel flow through thecarburetor venturi . On these models, a special aneroi dcavity insert is used in the fuel chamber to replace theadjustable metering rod assembly with filler spool, oraneroid-metering rod assembly, formerly used .

FIXED JET MAIN METERIN GRODS (2)MAIN METERING FIXED JE T

JETS (2 )

A . P .T . M E T E R I N G R O D A S S E M B L I E S [ M o d i f i e d D e s i g n

MAIN WELL AI RBLEEDS

MAIN POWER PISTON f r' THROTTLE VALVE S(DOWN) MAI NPOWER PISTON I MAIN M ETER I N GSPRING VACUUM METERING ROD SPASSAGE JETSMAIN METERING SYSTE M

F A C TO RY - M ETER I NGADJUSTMENT SCREW


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A DJ US TA BL E P A RT THR OTT L E FEA TUR EOn modified Quadrajet carburetors that use sepa-rated main wells, the A .P .T. adjustment consists of apin pressed in the side of the power piston which ex-

tends through a slot in the side of the piston well . Whenthe power piston is down (economy position), the pinstops on top of a flat surface of the adjustment screwlocated in a cavity next to the power piston (See Figure1 8) . The adjustment screw is held from turning by atension spring beneath the head of the screw .

During production flow test, the adjustment screw i sturned up or down which, in turn, places the taperedmetering rod at the exact point in the metering jet orificeto obtain the desired air/fuel mixture ratio to meetexhaust emission requirements .

N O T I C E : The A.P.T. screw is preset at thefactory and no attempt should be made t ochange this adjustment in the field . If a floatbowl replacement is required during carbu-retor service, the new service float bowl as-sembly will be supplied with the adjustmentscrew preset as required .

POWER SYSTEMThe power system in the Quadrajet carburetor pro-vides extra mixture enrichment to meet power require-ments under heavy engine loads and high-speed opera-

tion. The richer mixtures are supplied through th emain metering system in the primary and secondar ysides of the carburetor (Figure 19) .

The fuel mixture is enriched in the two primary boresthrough the power system . This consists of a vacuumoperated power piston and a spring(s) located in a cyl-inder connected by a passage to intake manifold vacuum .The spring(s) under the power piston apply an upwar dforce against manifold vacuum force tending to pull thepiston downward .

During part throttle and cruising ranges, manifol dvacuum is sufficient to hold the power piston dow nagainst spring force so that the larger diameter of th eprimary metering rod tip is held in the main meteringjet orifice to provide leaner mixtures during these periodsof engine operation . However, as the engine load i sincreased to a point where extra mixture enrichment i srequired, the power piston spring force overcomes thevacuum pull on the power piston and the tapered ti pof the primary metering rod moves upward in the mai nmetering jet orifice. The smaller diameter of the meteringrod tip allows more fuel to pass through the main meter-ing jet and enrich the fuel mixture to meet the adde dpower requirements. As engine load decreases, the mani-fold vacuum rises and extra mixture enrichment is n olonger needed . The higher vacuum pulls downward o nthe power piston against spring force which moves the

Figure 19 Power Systemjet orifice returning the fuel mixture to normal econom yranges .

Dual power piston springs are used beneath the powerpiston in the piston bore of some 4MV-4MC Quadrajetmodels (Figure 19) . A smaller diameter power pistonspring seats in the center of the piston and bottom son the float bowl casting . The spring is used to controlpower enrichment during light engine loads . A largerdiameter spring surrounds the smaller inner spring an dexerts additional pressure on the bottom of the powe rpiston to provide efficient mixture ratios at heavier engineload conditions. The dual power piston spring feature on models so equipped, assists in providing improvedfuel control of air/fuel mixture ratios to meet emissionand power requirements of the engine .

POWER SYSTEM - MECHANICA LOPERATIONSome Quadrajets have a mechanical power enrichmentsystem in addition to the vacuum enrichment feature .This provides accurate control of fuel mixtures at high

engine speeds and load and yet allows the use of vacuu menrichment for improved fuel control during mediu mengine loads .

The mechanical enrichment is controlled by a stempressed into the base of the power piston which extend sinto the throttle body. The stem is operated by a leve rwhich is hinged to the throttle body casting and a ca mon the center of the throttle shaft . When the throttlevalves are opened to a pre-determined point, the ca mon the throttle shaft forces the lever upward until i tcontacts the stem on the power piston and pushes th ecomplete piston assembly upward against engine vacuum(Figure 20) . This, in turn, lifts the metering rods placing the smaller diameter of the metering rods inthe main metering jets for positive mixture enrichmen tat greater throttle valve openings .

The power piston has a trapped spring which limitsthe travel of the piston during vacuum operation . The

AIR VALVE S(OPE N I

METERING MAIN -SECONDARYDISCS DISCHARGE THROTTLE VALVE SMAIN NOZZLESMETERINGRODS

POWER PISTON MAI NSPRING METERINGVACUUM JETSPASSAGEPOWER SYSTEM


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d9D-5March 198 1Page 14

Figure 21 Mul ti p le S tage Po w er Syste m

POWERPISTON (UP

M E C H A N IC A L P O W E R S Y S T E MPOWER SYSTE M

MAI NMAIN POWE RAUXILIARY METERING PISTON (UP )METERING ROD I) R O O S 1 21

SECONDARY METERING ROOS (2 )ACCELERATORWELL & TU B EACCELERATING WELL POR T

METERIN GROD LEVER

Figure 20 Mechanical Po w er Enr ichm en twasher on the power piston stem . The washer is re-tained with a C clip located in a groove on the powe rpiston stem .

During high engine vacuum, the power piston sprin gis compressed and the fluted washer and piston areseated at the bottom of the power piston cavity . Asengine load increases, vacuum drops and the powe rpiston moves upward against spring tension until th e C clip seats against the fluted washer. No more enrich-ment will take place until the pin in the power pistonis contacted by the mechanical enrichment lever . Asthe throttle valves are opened further, the complet epower piston assembly is forced upward placing th esmaller diameter of the metering rods in the jets fo rmaximum enrichment at higher engine speeds and loads.

When engine load and speed is decreased, the powerpiston will return to the down position, seating th efluted washer and piston in the bottom of the powerpiston cavity as high engine vacuum compresses th epower piston spring. This returns the metering systemto leaner fuel mixtures for light engine loads .MULTIPLE STAGE POWER ENRICHMENT

In some modified Quadrajet carburetors, a multipl estage power enrichment system, consisting of two powe rpistons (Figure 21), is used for more sensitive control ofair/fuel ratios during light duty engine power require-ments while providing for richer mixtures durin gmoderate to heavy engine loads .

AUXILIARY POWER PISTO NAn auxiliary power piston and single metering rodassembly, located in front of the main (primary) powe rpiston, is used for light duty power requirements . Onlight throttle opening when manifold vacuum drops t o

a predetermined point, the spring force under the auxiliarypiston overcomes the vacuum pull and raises the piston

which lifts the single metering rod out of a fixed meteringjet. This provides partial fuel enrichment for light dutyengine loads .

MAIN (PRIMARY) POWER PISTONDuring moderate to heavy engine loads when a furthe rdrop in manifold vacuum occurs with increased throttl eopening, the main (rear) piston spring force overcome sthe vacuum pull and raises the piston which lifts th e

two metering rods out of the metering jets for additiona lfuel enrichment for heavy duty power requirements .The multiple stage (two piston) power enrichmen tsystem is specifically calibrated for the power require-

ments of each engine by controlling spring rates of eachpiston. The system requires no adjustment in the field ;however, the main (rear) power piston and meteringrod assemblies and the auxiliary (front) power pisto nand metering rod assembly are removable for norma lcleaning and service replacement as needed .

N O T I C E : The main (rear) and auxiliary(front) power piston springs must NOT b einterchanged . To prevent mixing of powe rpiston springs at time of carburetor disas-sembly, lightly wrap a piece of masking tap earound the auxiliary power piston spring foridentification . Then, on reassembly, remov ethe tape and install the spring in the fron tlocation beneath the auxiliary power pisto nwith single metering rod .

P O W E R S Y S T E M -TURBOCHARGER APPLICATIONSSome modified Quadrajet models are designed forTurbocharger applications . The power system in thes emodels operates in the same manner as previously

described except for one important difference . Thevacuum supply, directed to the underside of the powe rpiston, is controlled externally by a Turbocharger Vacu-


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Figure 22 Secondary System - Typica lurn Bleed Valve . The vacuum signal is routed to th ecarburetor through a hose from the bleed valve to a nexternal tube located on the side of the carburetor .

The power system operates as follows :During part throttle and cruising ranges, engine loadis light and vacuum, from the Turbocharger Vacuum

Bleed Valve, is sufficient to hold the power piston dow nagainst spring force and the larger diameter of themetering rod tip is held in the metering jet for leanermixtures .As engine load is increased to a point where extrafuel enrichment is required and the intake manifold is

pressurized by the exhaust gas driven Turbocharger the vacuum bleed valve switches and reduces vacuumto the power piston to zero . At this point, spring forc eoperating on the power piston lifts the main meterin grods for increased fuel delivery .

The remote power enrichment feature, through thevacuum bleed valve, provides richer mixtures duringheavy engine loads and wide-open throttle power re-quirements when the intake manifold is pressurized bythe Turbocharger at a time when manifold vacuum i shigh enough tending to operate the power piston in th enormally lean position . In this way, the power syste mcontrols fuel metering during light and heavy powe rrequirements .

SECONDARY SYSTEMThe primary stage of the Quadrajet carburetor pro-vides adequate air and fuel for low speed operation .

to meet engine demands . The secondary stage of thecarburetor provides the additional air and fuel throug hthe secondary throttle bores for power and performanc erequirements .

The secondary stage has a separate and indepen-dent metering system (Figure 22) . It consists of twolarge throttle valves connected by a shaft and linkageto the primary throttle shaft . Fuel metering is controlledby a spring-loaded air valve, secondary metering orificeplates, secondary metering rods, fuel wells with blee dtubes, fuel discharge nozzles, accelerating wells an dtubes. These are used to modify fuel flow characteristicsfor exact air/fuel calibration .

The secondary metering system supplements fue lflow from the primary stage and operates as follows :When the engine reaches a point where the primarybores cannot meet engine air and fuel demands, a leve ron the primary throttle shaft, through a connecting

link to the pick-up lever on the secondary throttle shaft ,begins to open the secondary throttle valves . This occursonly if the choke has warmed the thermostatic coi lsufficiently to release the secondary throttle valve lock -out lever (if used) .

As the secondary throttle valves open, engine mani-fold vacuum (low pressure) is applied directly beneaththe air valves . Atmospheric pressure on the top of th eair valves forces the air valves open against spring an dair valve dashpot forces, provided the choke coil haswarmed sufficiently to release the air valve lockout lever,if used. This allows air to pass through the secondarybores of the carburetor .

On most models, accelerating wells are used to supplyfuel immediately to the secondary bores . This prevent sa momentary leanness until fuel begins to feed from th esecondary discharge nozzles. When the air valves begi nto open, the upper edge of each valve passes the ac-celerating well ports (one for each bore) . As the edge sof the air valves pass the ports, they are exposed tomanifold vacuum and immediately feed fuel from th eaccelerating wells located on each side of the fuel cham-ber. Each accelerating well has a calibrated orific ewhich meters the fuel supplied to the well from th efuel chamber. Some models have the accelerating wel lports located beneath the front edge of the air valv einstead of above. These begin to feed fuel to the secon-dary bores almost instantly after the secondary throttl evalves open and before the air valves begin to open .This type of porting is used on some models wher eadded enrichment is needed during cold operationwhen the air valve is locked closed, and also provide san earlier cut-in of fuel from the ports than the model swhich have the port located above the valves . The useof either type of porting is dependent upon engine fueldemands.

The secondary main discharge nozzles (one for each

SECONDAR YACCELERATO RWELLS & TUBES

SECONDARY METERIN GORIFICE PLATES SECONDAR YDISCHARG ENOZZLE SSE CO NDARY SYSTE M (Typical )


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9D-5March 198 1Page 1 6

and above the secondary throttle valves . The nozzles being located in a low pressure area, feed fuel as follows :

As the secondary throttle valves are opened, atmos-pheric pressure opens the air valves . This rotates aplastic eccentric cam attached to the center of the airvalve shaft. As the cam rotates, it lifts the secondar ymetering rods out of the secondary orifice plates throughthe metering rod hanger which follows rotation of th eeccentric cam .

Fuel flows from the fuel chamber through the secon-dary metering orifice plates into the secondary mainwells where it is mixed with air from the secondar ymain well air bleed tubes . The air emulsified fue lmixture travels from the main wells through the secon-dary discharge nozzles where it sprays into the secondarybores . Here the fuel is mixed with air traveling throughthe secondary bores to supplement the air/fuel mixturedelivered from the primary bores and goes on into th eengine as a combustible mixture .

As the secondary throttle valves are opened furthe rand engine speeds increase, air flow through the secon-dary side increases and opens the air valves to a greate rdegree which, in turn, lifts the secondary meteringrods further out of the orifice plates. The metering rodsare tapered so that the fuel flow through the secondarymetering orifice plates is directly proportional to ai rflow through the secondary carburetor bores . In thi smanner, correct air/fuel mixtures through the secondar ybores are controlled by the depth of the metering rod sin the orifice plates .

The depth of the metering rods in the orifice plate sin relation to the air valve position are factory adjuste dto meet air/fuel requirements for each specific engin emodel no change in this adjustment should be mad ein the field. Also, many models include a tang on theair valve lever which contacts a stop on the air hor ncasting to control carburetor air flow capacity.

METERING RODS - PRIMARYThere are two types of primary main metering rodsused in the Quadrajet carburetors . 1968 and late rmodels use a rod which has a double or multiple tape rat the metering tip. The 1967 and earlier models have asingle taper at the metering tip .

Both rod types use a similar two-digit numberingsystem . The number indicates the diameter of th emetering rod and is the last two digits of the part num-ber. The 1968 and later models will have a letter stampedon the rod after the two-digit number .METERING RODS - SECONDAR Y

The secondary rods are coded with a two-letter sys-tem which corresponds directly to the part number .

(See Delco Bulletin 9A-100 for a complete descrip-tion and listing of metering rods) .

There are other features incorporated in the secondarymetering system as follows :1. The secondary main well air bleed tubes exten d

downward into the main fuel well below normal fue llevel . These bleed air into the fuel in the secondar ywells to quickly emulsify the fuel with air for goodatomization and improved fuel flow from the secondarynozzles .

2. The secondary metering rods may have a milledslot at the larger diameter of the metering tip . Thepurpose of the slots is to ensure an adequate supply offuel in the secondary main wells when the air valves ar ein the closed position . At this point, the metering rodsare nearly seated against the metering orifice plates .The slot in the rod is adjacent to the orifice plate an dallows a small amount of fuel to pass between the meter -ing rod and metering disc. During extreme hot engineidle or hot soak, the fuel could boil out of the secondar yfuel wells. The milled slot allows enough fuel to by-pass the orifice plate and keep the main fuel wells ful lof fuel . This ensures adequate fuel supply in the mai nwells at all times to give immediate fuel delivery fromthe secondary discharge nozzles .

3. Some applications use secondary discharge nozzlesthat incorporate a vertically drilled cross hole locatedabout half way down the length of the nozzle . Thehole serves as an additional air bleed to assist air/fue lmixture distribution passing through the secondarydischarge nozzle.

4. A baffle plate, extending into each secondary bore ,is located just below the air valves on all models . Thebaffle extends up and around the secondary discharg enozzles to provide equal fuel distribution, as near a spossible, to all engine cylinders at lower air flows .

5. On some models, an integral baffle is added t othe bottom side of the secondary air valve . The baffleimproves mixture distribution from the secondary sideat higher air flows .6. An air horn baffle is used on some models t oprevent incoming air from the air cleaner reacting onthe secondary main well bleed tubes . The baffle is lo-cated adjacent to the secondary well bleed tubes an dextends above the air horn between the primary an dsecondary bores . This prevents incoming air from forcingthe fuel level down in the secondary wells through th ebleed tubes and prevents secondary nozzle lag on heav yacceleration .7. Some models use notched secondary air valves t oreduce the vacuum signal at the nozzles for leaner air

fuel mixture ratios during initial air valve opening . Theleaner mixtures assist in meeting emission requirement sand also improve throttle response when operating athigh altitudes .AIR VALVE DASHPOT

The secondary air valves use an air valve dashpot


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Figure 23 Air Valve Dashpot Design sfeature to control opening rate of the air valve . Thisprevents an uncontrolled air valve opening rate whic hresults in an instantaneous air rate change and a lag-ging fuel rate change as the secondary throttlevalves are opened . The dashpot, through linkage, con-trols opening of the air valves to provide a smoot htransition to secondary system operation .

There are two different types of air valve dashpot sused in the Quadrajet carburetor :The early type dashpot (Top of picture, Figure 23 )consists of a piston which operates in a fuel well adjacentto the float bowl . The piston stem is connected to th eair valve through a link and lever assembly . As the air

valves open, the dashpot piston is pulled upward forcin gfuel to flow between the side of the piston and fue lwell which retards the air valve opening . A rubbe rwasher attached to the piston stem acts as a check valve .During upward movement of the piston, the rubbe rwasher seats and forces all fuel to flow around the piston .When the air valve closes, the check valve unseats an dallows fuel to also pass through the center of the pis -ton allowing the air valves to return closed rapidly .

The late type air valve dashpot (Lower picture, Figure23) operates off of the front choke vacuum break dia-phragm unit. The secondary air valve is connected tothe choke vacuum break unit by a rod, to control theopening rate of the air valve. Whenever manifold vacuumis sufficiently high, the vacuum diaphragm is seated ,plunger retracted, against spring load . At this point ,the vacuum diaphragm link is in the forward end of theslot in the air valve lever, or in the rear of the slot i nthe vacuum diaphragm plunger, and the air valves are

Figure 24 Pump Syste mDuring acceleration or heavy engine loads when th esecondary throttle valves are opened, the manifol dvacuum drops . The spring located in the vacuum dia-phragm unit overcomes the vacuum force and move sthe plunger stem outward. This action allows the air

valves to open . The opening rate of the air valves i scontrolled by the calibrated restriction in the vacuuminlet of the diaphragm cover and the valve closing spring .The dashpot action, due to this restricted vacuum flowand spring force, provides the required delay in ai rvalve opening needed until sufficient fuel flows fromthe secondary discharge nozzles .ACCELERATING PUMP SYSTEM

During quick accelerations when the throttle i sopened rapidly, air flow through the carburetor bore sand intake manifold vacuum change almost instanta-neously. However, the fuel which is heavier, tends tolag behind causing a momentary leanness . To preven tthis, the accelerator pump system is used to provid ethe extra fuel necessary for a smooth transition in engineoperation during this period .

The accelerating pump system is located in the primarystage of the carburetor. It consists of a spring-loade dpump plunger and pump return spring (operating in afuel well), fuel passage, discharge check ball, retainer and pump jets, one in each bore (Figure 24) .On most late Quadrajet models, an expander (garter )

spring is used in the pump cup for constant pump cu pto pump wall contact . The pump cup is of the floatingdesign; i .e ., the up and down movement of the cup o nthe plunger head either seats to provide a solid chargeof fuel on the down-stroke, or unseats on the fillingof the pump well (up-stroke) . The cup remains unseatedwhen there is no pump plunger movement which allows

AIR VALVE DASH POT OPERATION (EARLY )AIR VALVE SHAFT LEVE R

C L O S E DOPEN

ACTUATING RO DVA C UUM B R E A KDIAPHRAGM ASSEMBL Y

AIR VALVE DASHPO OPERATION (LATE )

AIR VALVE C LOSING SPRIN Gn L E V E Rr /L IN K

PISTON ~~. 'HINGES T E M rlu ~~ P IN

P U M PL E V E R

D I S C H A R G Eyy PASSAG ED I S C H A R G EB A L L R E T A I N E R

PUMP DISCHARG ERETURN CHECK BALLP U M P S P R I N GP L U N G E R E X P A N D E RS P R I N G

PUMP SYSTE M


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Delco

CHOKE ENRICHMEN TFUEL CHANNE LCHOKE VALVE

_ CHOKE ROD /-`

I r-)F A S T ID L E C A MV A C U U M BREA KLEVER AND TANG

VACUUM BREA KAS S EMBL

'-~--UNIOADER T AN G SFAST IDLE SCR E W

P

4M-2 5

Figure 25 Pump Stem Seal & Retaine rThe pump plunger is operated by a pump lever onthe air horn which is connected directly to the throttl e

lever by a rod .When the pump plunger moves upward in the pumpwell as happens during throttle closing, fuel from th efuel chamber enters the pump well through a vertica l

slot located near the top of the pump well . It flow spast the unseated pump cup to fill the bottom ofthe pump well and pump discharge passage .

When the primary throttle valves are opened, thepump rod and lever forces the pump plunger downward .The pump cup seats instantly and fuel is forced throughthe pump discharge passage where it unseats the pumpdischarge check ball and passes on through the passag eto the pump jets where it sprays into the venturi areaof each primary bore .

It should be noted the pump plunger is spring loaded .The upper duration spring is balanced with the bottompump return spring so that a smooth, sustained chargeof fuel is delivered during acceleration. Selection of theduration spring by the factory is used to control th edifferences in rate of movement between the pumplinkage and the plunger head for correct pump fue ldelivery .

The pump discharge check ball seats in the pumpdischarge passage during upward motion of the pumpplunger so that air will not be drawn into the passag eand prevent proper pump fill .During higher air flows through the primary bores ,a vacuum exists at the pump jets . A passage, locate d

just behind the pump jets, leads to the top of the ai rhorn to vent the pump fuel circuit outside the carburetorbores . This acts as a suction breaker so that when thepump is not in operation, fuel will not be pulled out ofthe pump jets into the venturi area . This insures a fullpump stream when needed and prevents any fuel pull-over from the pump discharge passage .

Figure 26 Choke System-4MVIn order to keep evaporative emission levels to aminimum, later model M4ME-M4MC models have a

pump plunger stem seal and retainer located in the ai rhorn . In the event of service repair or cleaning of th ecarburetor, a new seal and retainer should be installed(See Figure 25) .

CHOKE SYSTE MThe Quadrajet choke valve is mounted in the airhorn located above the carburetor primary venturi . A

closed choke valve provides the correct air/fuel mixtur eenrichment to the engine for good cold engine startin gand when partially open smooth running during th ewarm-up period . The secondary throttle valves, or airvalve on some models, are locked closed until the engineis thoroughly warm and the choke valve is wide open .

The Model 4MV choke system (Figure 26) consists ofa choke valve, vacuum break diaphragm assembly thermostatic coil-mounted on the intake or exhaustmanifold, fast idle cam, and connecting linkage .

Heat from exhaust gases is used for control of thermo-static coil temperature . Choke operation is controlle dby a combination of intake manifold vacuum, the off -set choke valve, temperature, and throttle position .

The thermostatic coil is calibrated to hold the chokevalve closed when the engine is cold.The choke system operates as follows :When the engine is cold, prior to starting, depressin g

the accelerator pedal to the floor opens the carburetorthrottle valves. This allows the fast idle cam followerlever to clear the steps on the fast idle cam . At thispoint, torque from the thermostatic coil closes the chokevalve and rotates the fast idle cam so that the cam fol-lower lever comes to rest on the highest step of th efast idle cam . (This opening of the throttle valves also


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pumps a priming mist of fuel through the pump jetsinto the primary throttle bores to aid starting) . Duringcranking, engine vacuum below the choke valve pull sfuel from the idle system and main discharge nozzles .This provides adequate enrichment for good cold starts .Some Quadrajet models use a choke enrichment fuelsystem to supplement fuel feed from the primary mai n

discharge nozzles for good cold engine starting .Two calibrated holes, one in each primary bore, ar elocated in the air horn just BELOW the choke valv eto supply added fuel for cold enrichment during thecranking period. The extra fuel is supplied throug hchannels which lead to the secondary accelerating well

pickup tubes to allow fuel at closed choke to be draw nfrom the secondary accelerating wells located in the floatbowl chamber, (As shown in Figure 26) . Also, duringwarm engine operation, the two calibrated holes in th eair horn feed a small metered amount of fuel at higherair flows to supplement fuel flow in the primary bore sto provide the extra fuel needed at higher engine speeds .

As mentioned earlier (See Main Metering System) other Quadrajet models use the fuel pull-over enrich-ment system . This system is similar to the chok eenrichment fuel system except that two calibrated holes ,one in each primary bore, are located in the air hor njust ABOVE the choke valve to supply added fuel duringhigher carburetor air flows. The calibrated holes, locatedabove the choke valve, do not feed fuel at closed chok eduring the engine cranking period .

When the engine starts and is running, manifol dvacuum is applied to the vacuum break diaphragmunit mounted on the side of the float bowl . This move sthe diaphragm plunger in until it strikes the rear cover thereby opening the choke valve to a point where theengine will run without loading or stalling . (This is calledthe vacuum break position) . At the same time, thefast idle cam follower lever on the end of the primarythrottle shaft will drop from the highest step on th efast idle cam to a lower step when the throttle is opened .This gives the engine sufficient fast idle and correctfuel mixture for running until the engine begins t owarm up and heat the thermostatic coil .

As the engine continues to warm up, heat graduallyrelaxes tension of the thermostatic coil to allow thechoke valve to continue opening through air pressur epushing on the offset choke valve and the weight of thelinkage pulling the choke valve fully open at which pointthe engine can run at normal air/fuel mixtures .The fast idle cam has graduated steps so that the fas tidle speed is lowered gradually during the engine warm-up period . The fast idle cam movement (and ste pposition) is a function of choke valve position . Whenthe engine is warm and the choke valve is completel yopen, the fast idle cam follower will be off the step sof the fast idle cam . At this point, the idle speed screw

Figure 27 Secondary Throttle Valve LockoutS E C O N DAR Y T H R O T T L EVALVE LOCKOUT

A secondary throttle valve lockout feature is used o nmany Quadrajet models to prevent opening of the secon-dary throttle valves during cold operation when wide -open accelerations could cause possible engine damag eor excessive wear (Figure 27) . On these applications ,a lockout lever, located on the float bowl, is weighte dso that a tang on the lower end of the lever catches alock pin on the secondary throttle shaft and keeps th esecondary throttle valves closed. As the engine warmsup, the choke valve opens and the fast idle cam drops .When the engine is thoroughly warm, the choke valv eis wide open and the choke coil pulls the intermediat echoke lever completely down and the fast idle ca mdrops down so that the cam follower is completely of fthe steps of the cam. As the cam drops the last fewdegrees, it strikes the secondary lockout lever and pushe sit away from the secondary valve lockout pin . Thisallows the secondary throttle valves to open and operat eas described under the Power System .AIR VALVE LOCKOUT

Some Quadrajet carburetors incorporate an air valv elockout feature (Figure 28), instead of the secondar ythrottle valve lockout, whereby the air valves are locke dclosed until the engine is thoroughly warm and thechoke valve is wide open . An air valve lockout lever ,mounted on the air horn, is so weighted that a tang o nthe lever catches the upper edge of the air valve an dkeeps the air valves closed when the choke valve is closed.When the thermostatic coil warms up it moves the chok evalve toward the open position, the end of the chokerod moves upward and strikes a tang on the air valvelockout lever. As the choke rod moves up to the end o fits travel, it pushes the lockout tang upward and un-

C H O K E S Y S T E M ( W IT H S E C O N D AR YLOCKOUT FEATURE ) 4M-2 6

SECONDAR YOCKOUT LEV E RFAST IDLE CAMFOLLOWER


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Delco

Figure 28 Air Valve LockoutAlso, some early model 4MV carburetors use a vacuu mbreak modulating spring and split choke spring (Figur e28 ) . The vacuum break modulating spring allows th evacuum break (choke valve position) to vary accordin gto ambient temperature. The vacuum break modulatingspring, connected to the vacuum break link, allow svarying choke openings depending on the closing forc e

of the thermostatic coil . As the closing force of the coi lincreases (cool weather), the link is allowed to move i nthe slotted lever until the modulating spring overcomesthe coil force, or the link is in the end of the slot. Thisresults in less vacuum break during cooler weatherand more vacuum break during warmer weather .

The split choke feature operates during the last fewdegrees of choke thermostat rotation . The purpose is tomaintain the fast idle speed long enough to keep th eengine from stalling, but allow the use of a choke coi lwhich lets the choke valve open quickly. The operationof the split choke feature is controlled by a torsion springon the intermediate choke lever shaft . As explainedearlier, air pressure action on the offset choke valv etends to force the choke valve open against tension o fthe choke thermostatic coil . In the last few degree sof thermostatic coil opening motion, a tang on the inter -mediate choke lever contacts the end of the torsio nspring. This keeps the fast idle cam follower lever o nthe last step of the fast idle cam longer to maintai nfast idle until the engine is thoroughly warm . The springworks against the thermostatic coil until the coil is hotenough to pull on the intermediate choke lever and over -come the torsion spring tension . The torsion sprin gmust be placed in the specified notch in the vacuumbreak mounting bracket for application used .On all 4MV models, the choke system is equippe dwith an unloader mechanism which is designed to par-tially open the choke valve, should the engine becomeloaded or flooded. To unload the engine, the acceleratorpedal must be depressed so that the throttle valves areheld wide open. A tang on a lever on the choke side of

Figure 29 Spring Assist Choke Closing Syste mthe primary throttle shaft contacts the fast idle camand through the intermediate choke shaft forces thechoke valve slightly open . This allows extra air to ente rthe carburetor bores and pass on into the engine mani-fold and cylinders to lean out the fuel mixture so tha tthe engine will start .CHOKE SYSTEM WITH SPRING ASSIS TCHOKE CLOSING SYSTE MSome 4MV carburetors use a spring assist chok eclosing system (Figure 29) . The assist spring is of thetorsion type and is added to the intermediate chokeshaft. It exerts pressure on the vacuum break lever t oforce the choke valve toward the closed choke position .The tension of the torsion spring is overcome by th echoke thermostatic coil located on the engine manifol dwhich, during the engine warm up period, will pull thechoke valve open . The addition of a torsion springassists in closing the choke valve to ensure good enginestarting when the engine is cold .

Along with the choke closing assist spring, certain4MV models use the fast idle cam pull-off feature.When the engine starts and is running, manifoldvacuum is applied to the vacuum break diaphragm an dthe diaphragm plunger moves slowly inward to ope n

the choke valve . As this happens, a tang on the plunge rcontacts the end or tail of the fast idle cam to pull -off the cam from the high step to the lower secon dstep setting .A slight change in the method of vacuum break ad-justment is required on these models that use the fas t

idle cam pull-off' feature . (See Adjustment Proceduresin the 9D-5 Section of the Delco Carburetor Parts an dService Manual 9X) .CHOKE SYSTEM WITH VACUUM BREA KCLOSING ASSIST SPRIN G

Other 4MV models have the choke closing assis t

CHOKE SY STEM 4M-27

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4M-28


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F igu re 30 V acuum B reak C los ing Ass is t Spr in gspring located on the vacuum break plunger stem re -placing the torsion spring located on the intermediat echoke shaft (Figure 30) .

The choke closing assist spring aids in closing th echoke valve along with tension from the remote chokethermostatic coil for improved engine starting. Thechoke closing assist spring only exerts pressure on thevacuum break link to assist in closing the choke valveduring engine starting. When the engine starts and thechoke vacuum break diaphragm seats, the closing springretainer hits a stop on the plunger stem and the assistspring no longer exerts pressure on the choke valve .

The vacuum break diaphragm plunger is slotted toallow for free travel of the air valve dashpot link . Theslot is moved from the air valve shaft lever to the vacuumbreak plunger . A change in air valve dashpot adjust-ment procedure is required. (See Adjustment Proceduresin the 9D-5 Section, Delco Carburetor Parts and ServiceManual 9X) .CHOKE SYSTEM WITH DELAYE DVACUUM BREAK FEATUR E

To delay the choke valve from opening too fast, some4MV models use a delayed vacuum break system . Aninternal delay valve is included inside the diaphragmunit (Figure 31) .

The delay feature operates as follows :When the engine starts, vacuum acting on the interna ldelay valve bleeds air through a small hole in the valv ewhich allows the vacuum diaphragm plunger to mov e

slowly inward . This gives sufficient time to overcomeengine friction and wet the engine manifold to preventa lean stall . When the vacuum break diaphragm is full yseated, which takes a few seconds, the choke valv ewill remain in the vacuum break position until th eengine begins to warm and relax the thermostatic coi llocated on the exhaust crossover in the intake manifold .

F igu r e 3 1 De lay ed V acuum Br eak F ea t u re

F igure 32 V acuum Break B uck ing Sprin gapplications have a separate vacuum delay tank addedto the system . This is connected in series to a secondvacuum tube on the vacuum diaphragm unit to furtherdelay the choke vacuum break diaphragm operation .

The delay valve in the choke vacuum diaphragm uni tis designed to pop off its seat and allow the diaphrag mplunger to extend outward, when the spring force againstthe diaphragm is greater than the vacuum pull . Thi swill give added enrichment as needed on heavy accelera-tion during cold drive-a-way by allowing the choke coilto slightly close the choke valve . Some 4MV modelsuse a calibrated restriction in the vacuum inlet to th evacuum break diaphragm unit in place of the interna ldelay valve . Similar to the internal delay valve, the cali-brated restriction delays the supply of vacuum to th ediaphragm unit to retard opening of the choke valve forgood engine starting .

CHOKE SYSTEM WITH VACUU MBUCKING SPRIN GA spring-loaded plunger is used in the vacuum brea kunit on some 4MV models (Figure 32) . The purpose ofthe spring, called a bucking spring , is to offset chok ethermostatic coil tension and balance the opening o fthe choke valve with tension of the choke coil . Thi s

AIR VALV ES H A F T L E V E RC H O K ER O D

CHOKE CLOSINGASSIST SPRIN GF A S T I D L E C A M

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V A C U U M B R E A K B U C K IN G S P R IN G


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In addition to the internal delay valve, some car9D-5March 1981Page 22

elco

enables further refinement of air/fuel mixtures becausethe coil, which senses engine and ambient temperatures ,will allow the choke valve to open gradually againstspring tension in the diaphragm plunger head . In otherwords, in very cold temperatures, the extra tensioncreated by the thermostatic coil will overcome the ten-sion of the diaphragm plunger (bucking) spring toprovide less choke valve opening with the result ofslightly richer mixtures . In warmer temperatures, thethermostatic coil will have less tension and, consequently ,will not compress 'the spring as much thereby giving agreater choke valve opening for slightly leaner mixtures .CHOKE SYSTEM WITH DUAL DELAYE DVACUUM BREAK SYSTE M

Other Quadrajet carburetors use a dual delayedvacuum break system consisting of front and rear vacuu mbreak diaphragm units (Figure 33) .The dual delayed vacuum break system operates a s

fol lows :During engine cranking, the choke valve is held close d

by the tension of the thermostatic coil . This restricts ai rflow through the carburetor to provide a richer startin gmixture .

When the engine starts and is running, manifold vacu-um is applied to both vacuum break units which ar emounted on the side of the float bowl . The front (pri-mary) vacuum break diaphragm opens the choke valveto a point where the engine will run without loading orstalling. As the engine is wetted and friction decreasesafter start, a delay valve in the rear (auxiliary) vacuu mbreak unit causes a delayed action to gradually ope nthe choke valve a little further until the engine wil lrun at a slightly leaner mixture to prevent loading .

A plunger bucking spring may be included on th ediaphragm plunger of the rear diaphragm unit (Seeexplanation, plunger bucking spring operation) .A clean air purge feature is added to the rea r

vacuum break diaphragm unit on some 4MV model susing the dual delayed vacuum break system (Figure 33) .A clean air bleed, added to the tube at the rear of therear vacuum break unit and located beneath a rubbe rcovered filter, purges the system of any fuel vapor sand dirt which possibly may enter the internal dela yvalve to disrupt operation . A change in adjustmentprocedure for setting the rear vacuum break is require don those models using the clean air purge feature . (SeeAdjustment Procedures, Section 5, of the Delco Car-buretor Parts and Service Manual 9X) .

CHOKE SYSTEM - 4MC (EARLY)The model 4MC carburetor differs from the 4M Vmodel in that the thermostatic choke coil is located i na choke housing, mounted directly to the side of th ecarburetor float bowl, instead of a remote chokecoil mounted on the intake or exhaust manifold .

Figure 33 Dual Delayed Vacuum Break Syste m

Figure 34 Early 4MC Choke Syste mOn 4MC models, engine vacuum is supplied throug han orifice in the choke housing which pulls heat fromthe manifold heat stove into the choke housing an dheat gradually relaxes choke coil tension . This allows

the choke valve to continue opening through inlet ai rpressure pushing on the offset choke valve and theweight of the choke linkage pulling the valve open .

On early model 4MC carburetors (Figure 34), th evacuum break diaphragm is mounted integrally with th echoke housing located on the float bowl . An adjustabl eplastic plunger is included as a part of the vacuu mbreak diaphragm . During cold operation after th eengine starts and is running, manifold vacuum appliedto the vacuum diaphragm pulls the diaphragm inwar dand the plunger strikes the vacuum break tang insidethe choke housing which, in turn, rotates the inter -mediate choke shaft and through connecting linkageopens the choke valve to the vacuum break position .

CHOKE SYSTEM - 4MC (LATE)The choke housing on later model 4MC carburetor s

DU L DEL YED V CUUM BRE K

P L UN G E R B U C K I N GSPR I N GTHERMOSTATI CCHOKE COI L

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CHOKE SHAFTAND LEVER f C H O K EVALV E

EARLY 4MC CHOK E SYSTE M


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Page 23

Figure 35 Later 4MC Choke Syste m(Figure 35) is also located on the float bowl, but with aseparately mounted vacuum break diaphragm unit .The vacuum diaphragm unit on some 4MC models in-cludes an internal delay valve for good control of chokemixtures during the engine warm-up period (see descrip-tion of Delayed Vacuum Break System, Figure 31) .

CHOKE SYSTEM - M4MC-M4MCASimilar to 4MC models, an integral choke housin gand an exhaust heated air thermostatic coil assembl yare mounted on the carburetor float bowl on M4MC-M4MCA models (Figure 36). A single or a dual vacuumbreak system is used for control of choke mixtures duringthe warm-up period . A special adjusting screw is in-cluded on the front vacuum break unit on all modifiedQuadrajet models. The vacuum break is adjusted byturning this screw to position a tang on the plungerthat contacts the vacuum break lever .

On some models, the front and/or rear vacuum brea kunits are delayed in operation by an internal delay valv e(see inset, Figure 36). The valve delays opening of thechoke valve a few seconds to allow the engine to ru non richer mixtures .

A clean air purge feature is used in either the fron tand rear vacuum break diaphragm units or in the rearunit only, depending upon carburetor application, toprevent dirt from plugging the internal delay valve . Afilter element is installed internally with a small blee dhole located in the end cover of the diaphragm unit .During engine operation, vacuum acting upon the dia-phragm pulls a small amount of filtered air through th ebleed hole in the end cover to purge the system of an yfuel vapors or dirt contamination which might be pulle dinto the internal delay valve located inside the diaphragmunit . During adjustment of the front and rear vacuu mbreaks, it will be necessary to plug the end cover ofthe vacuum break unit with tape, making sure to sea lthe small bleed hole .

Figure 36 M4MC-M4MCA Choke System (Typical )Also, a vacuum inlet check ball may be used in th evacuum inlet tube on the front and rear vacuum brea k

units. The purpose of the inlet check ball is to preven texcess dirt and vapor contaminants from plugging th esmall internal delay valve in the diaphragm unit i ncase of engine backfire or dieseling conditions .

The rear vacuum break unit on some application smay include a tension (bucking) spring in the diaphragmplunger head. (See previous explanation Plunger BuckingSpring Operation Page 21) .

On some M4MC-M4MCA models, the rear vacuu mbreak unit includes _a choke closing assist spring (SeeFigure 30 and previous explanation of closing assis tspring operation Page 20) .Some M4MC-M4MCA models incorporate the trap-ped stat thermostatic coil design whereby the en dof the thermostatic coil is installed in a slot in the coi lpick-up lever inside the choke housing . In this way

the coil is kept in contact with the pick-up lever atall times for prompt response to choke coil tension .

FAST IDLE CAM PULL-OFF FEATUR EA fast idle cam pull-off feature is included on

some M4MC-M4MCA carburetor applications. Manifoldvacuum to the rear vacuum break diaphragm is suppliedthrough a water temperature controlled thermal vacuu mswitch (TVS) and by an electrically operated time dela ysolenoid.

During cold operation when manifold vacuum i sapplied through the TVS and the solenoid to the rea rvacuum break diaphragm, the diaphragm plunge rmoves inward pulling on the vacuum break rod whic hrotates the vacuum break lever to pull-off the fastidle cam from the high step to the lower step setting .Thus, the cam pull-off feature prevents prolongedhigh idle speeds during the warm-up period .

FRONT VACUU MCHOKE BREAK ADJUSTIN GROD CHOKE / S C R E WFRONT VACUUM BREA KDIAPHRAG M(DELAY UNIT USE D O N-SOME MODELS

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CHOKE CLOSING ~ ~~ASSIST SPRING ~ THERMOSTATI C- COIL UE C O N D A R YLOCKOUT FAST IDLE INSIDE FAST IDLE TAN GL E V E R CAM CUP ADJUSTIN GFOLLOWER BAFFLE SCR E WFRONT VACUU MBREAK ADJUSTIN G

S C R E WFRONT VACUU MB R E A KDIAPHRAGM


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9D 5March 1981Page 26

is adjusted at the factory . No attempt shouldbe made to adjust the Idle Load Compensa-tor unless, in diagnosis, curb idle speed isnot to factory specifications . If adjustmentis necessary, refer to Service Manual forproper procedure s

MAJOR SERVICE OPERATIONS -ALL MODELSG e n e r a l

The procedures, below, apply to the complete over-haul with the carburetor removed from the engine .However, in many cases, service adjustments of indi-vidual systems may be completed without removingthe carburetor from the engine .

A complete carburetor overhaul includes disassembly ,thorough cleaning, inspection, replacement of all gas-kets diaphragms seals worn or damaged parts ,and service adjustment of individual systems, plus re -storing tamper resistant features where applicable.Disassem bly, C l ean ing , I nsp ection and Ad jus tm ent s

The following disassembly and assembly proceduresmay vary somewhat between applications due to specificdesign features. However, they will pertain basicallyto all Quadrajet models .

Figure 42 Typical Quadrajet Carbureto rDisassembly

N O T I C E : Before performing any service onthe carburetor, it is essential that the carbu-retor be placed on a holding fixture such a sBT-30-15 . Without the use of the holdin gfixture, it is possible to bend or nick throt-tle valves .

Idle Stop Solenoid Remova l1. Remove screw(s) securing the idle stop solenoi dbracket to float bowl and remove solenoid and bracke t

assembly. On some applications, the solenoid an dbracket assembly are an integral unit and serviced asa complete assembly. On some applications where thesolenoid is mounted in the bracket as a separate unit ,if solenoid replacement is necessary, bend back retainin gtabs on lockwasher; then remove large idle stop solenoidretaining nut and remove solenoid from bracket .

N O T I C E : Follow the above procedure toremove the A/C idle speed solenoid ,throttle closing dashpot, or throttle leveractuator . The idle stop solenoid, A/C idlespeed solenoid, throttle closing dashpot or throttle lever actuator should not be im-mersed in any type of carburetor cleanerand should always be removed before com-plete carburetor overhaul .

2. On 4MV models using either a Combinatio nEmission Control (C .E.C .) valve or idle stop solenoi dmounted on the carburetor :a) On C .E.C . models only, remove vacuum hos efrom the C .E.C . valve and vacuum tube on the floatbowl .b) Remove screw securing C .E.C . valve or idle stop

solenoid bracket to float bowl .N O T I C E : Do not remove the bracket forthe C.E.C . valve or idle stop solenoid fro mthe air horn assembly unless replacement o fthe bracket is necessary. If necessary to re-place the C .E.C . valve or idle stop solenoid follow procedure noted under Step 1, above .Do not immerse the C .E.C . valve assemblyor idle stop solenoid in any type of carbu-retor cleaner .

AIR HORN REMOVAL1 . If used, remove idle vent valve attaching screw ;then remove idle vent valve assembly . If thermostaticvent valve is used, remove dust cover, then removevalve . Care should be used not to bend or distort th ebi-metal strip .2. On those 4M V models using the vacuum operate d

vent switch valve, remove small screw from top of ven tvalve plunger stem .N O T I C E : Hold plunger stem with needle -nosed pliers to prevent turning and tearingof diaphragm . Remove vent valve cover screwand air horn screw and carefully lift coverfrom air horn . Remove cover gasket, ven tvalve assembly, and spring, noting positionof the vent valve for later reassembly . Re -move diaphragm retainer and diaphrag mfrom the air horn by carefully moving th ediaphragm stem back and forth .

AIR HOR NAIR VALV E

CH O K EVALVEPU M PLEVE R

PU MPS TE M

TYPICAL Q UADRAIET CARBURE TOR


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quadrajet service manual 1981

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