gt40webercarburetor.pdf

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GT40WeberCarburetor

Weber Carburetors Tuned. Finally.

Six months ago Ryan and I knew nothing about Weber Carburetors, other than the lookedcool !"ter s#ending countless hours reading e$ery thing we could get our hands on,

some $aluable guidance "rom other threads by #eo#le a lot smarter than us, and manymore hours working with the carbs, we can "inally say they are %ust about tuned We canstart it u#, it s#its a cou#le o" times, then settles down in less than ten seconds, a"ter

which it idles smoothly It accelerates well and only rarely s#its a"ter it is warmed u# It

runs steady and accelerates through the R&' range without noticeable hesitation or

stumbling

So much has been written about Webers that I am reticent to add more (onetheless a

write u# o" the issues we disco$ered and resol$ed will "ollow To those that ha$e li$edwith these carbs this is likely old school and o" little interest, but to someone that has

ne$er worked with Webers ho#e"ully this will be o" some bene"it Se$eral #osts

chronicling the #rocess will "ollow

There are lots o" good books &at )raden, Weber Carburetors, )ob Tomlinson*s Original

Weber Tech Manual and +aynes Weber Carburetor Manual are #erha#s the best knownI like Tomlinson*s book best because it has lots o" good ex#lanations o" how they work

and "ocuses on the I- carbs s#eci"ically

Weber Carburetors, Specs

The 44 I- carbs came installed on the .0/ crate engine It had been dyno tested by the

builder So our ex#ectation was that only some minor ad%usting would be needed to get

them tuned in )etween anuary and 'arch, /001, we s#ent hours ad%usting to no a$ail



+ere are the engine s#ecs "or re"erence2

.0/ ciCom#ression 1.3 2

Chamber $olume2 50 cc

Com# cam, hydraulic rollersart #ro head

Intake $al$es2 14

6xhaust $al$es2 50

+ere are the original carb settings, as su##lied by the engine builder2

44 I-Idle2 50

'ain .7

!ir 37

8enturi choke tube ./!ux 8enturi 47

Since our e""orts did not sol$e the tuning #roblems, in 'arch, /001 we decided to go

back to the basics and start o$er The "irst issue was the linkage

Weber Carburetors, Linkage. March, 2009

The bolt that holds the bell crank was not tight 9our thinking :how could they not ha$e

noticed that sooner; Good #oint We mistakenly assumed that the linkage had been #ro#erly assembled <#on tightening it the su##ort #ost, it no longer mo$ed So we

remo$ed it The bell crank had been installed u#side down, since the bearing sets slightly

below the sur"ace o" the bell crank on one side to clear the mounting #ost We reinstalledit and tightened it down (ow it turned easily This little "ix remo$ed much o" the =#lay*

in the linkage, but there was still some =#lay* remaining2 an issue that would come back

to haunt us later

(ext attention was turned to the two connecting arms that %oin the bell crank to the carbs

The ends had been cut o"" rather slo##ily at an angle ! >uick measurement con"irmed

that one was .?5 inch longer @See #ictureA This could a""ect the synchroniBation whenthe carbs #rogressed towards a more o#en throttle #osition Some time with a "ile e$ened

u# the ends and made the two e>ual in length !nti seiBe was used on the heim bolts

when it was reassembled

6ach carb has a se#arate throttle return s#ring We originally remo$ed the s#rings on the

two de#endent carbs @the ones not directly connected to the bellcrankA #er therecommendation in &at )raden*s book Weber Carburetors This reduced the resistance in

the system )ut we later #ut them back The extra s#rings were needed to com#ensate

"or the nine "oot throttle cable



)e"ore the linkage between the carbs was reconnected, the idle sto# screws on each carb

were set to %ust barely touch when the throttle #late was "ully closed The two #rimary

carb idle sto# screws will be used "or idle ad%ustment, and the remaining two on thede#endent carbs will #ro$ide a re"erence #oint "or "ully closed

The two arms connecting the bell crank to the carb linkage need to be as symmetrical as #ossible )y #lacing nylon washers between the heim %oint and the carb linkage the

symmetry can be ad%usted ne wants to assure that the linkage "ollows the same arc o"

tra$el on both sides so the "low through the carbs will be e>ual throughout the R&'range

!"ter we remo$ed the two carb return s#rings, we added a se#arate return s#ring to the

bell crank The amount o" s#ring tension could be easily ad%usted with this s#ring Welater remo$ed this s#ring when we disco$ered that it could skew the motion o" the bell

crank That is when we reinstalled the carb s#rings 'ore on that issue later

This little exercise taught us two im#ortant lessons2 -irst, don*t assume the carbs on acrate engine are #ro#erly installed Second, #ro#er linkage is absolutely critical on

Weber carbs

The #ictures show a cou#le o" be"ore shots and one a"ter shot with the bell crank and

connecting arms in #lace (ote that the arms are the same length and that they are #arallel to each other The return s#ring $isible in the #hoto was later remo$ed



Weber Carburetors, Floats. pril, 2009

The linkage reinstalled and throttle #lates set, we mo$ed on to the "loat chambers Wewere a bit a##rehensi$e about this %ob, ha$ing ne$er done it be"ore, but it turned out to be

"airly straight "orward and une$ent"ul We ordered carb rebuild kits, which included a

com#lete set o" gaskets in the e$ent any were damaged -ortunately we did not need tore#lace any gaskets

The to# o" each carb was remo$ed The "uel was drained @using a $acuum #um# "or bleeding brakesA and the carb bowl dried out with com#ressed air We wanted to make

sure there was not a trace o" dust or dirt

! sim#le tool was "abricated "rom a short #iece o" eighth inch aluminum rod to check the

"loat le$els ! notch at 0 mm and ./7 mm made it easy to check the "loat le$els in the

manner described in se$eral books, including &at )raden*s Weber Carburetors +ere is

where it got a bit interesting



ne o" the "loats had been set at . mm Two were set at 1 mm nly one was #ro#erly

set at 0 mm We made sure all "our were exactly at 0 mm be"ore reassembling them&erha#s the . mm carb was running a bit lean contributing to our #roblems

While the carb to#s were o"", the idle %ets, mixture screws and main %ets were all remo$edand #assages blown out with com#ressed air This hel#ed assure there was no dirt or

dust 'ake sure the rings on the idle %ets and mixture screws don*t get damaged We

ordered some extra rings, since some had de"ormities and nicks

The gas #edal was ad%usted to align its #osition in relationshi# to the brake #edal The

cable was read%usted so when the gas #edal was "ully de#ressed the throttle #lates were

%ust shy o" "ully o#ened

Weber Carburetors, Throttle Sha!t. May, 2009



Se$eral weeks o" lousy weather ke#t us "rom dri$ing the GT -inally the sun came out on

a weekend in 'ay and we were able to resume the #ro%ect Time was s#ent ad%usting the

air "low and the mixture The air "low on the carbs balanced within a needle width o"each other We thought we had the carb calibration nailed Ryan and I took it "or a "orty

mile dri$e and it ran with only an occasional s#it It accelerated relati$ely smoothly but

with a bit o" hesitation u# to 4700 R&', idled rock steady, and ran well at steady s#eeds below .000 r#m The next day we took it "or another dri$e The "irst twenty miles all

was well Then it started coughing and s#itting across the R&' range "or no a##arent

reason It had lost the balance dialed in the day be"ore

-rustrated, we #arked it and "orgot about it "or a cou#le o" days (ext $isit to the garage

we made an interesting disco$ery

The throttle #lates are mounted on a sha"t that has linkage on either end, held in #lace

with a nut We noted that the linkage could be wiggled on both ends o" each carb, to

$arying degrees &at )raden*s book, Weber Carburetors, states that the nuts should not

be o$er tightened since o$ertightening will cause the throttle to bind

:Tighten the throttleDsha"t nuts lightly I" you o$ertighten them they will causethe throttle bellcrank to bind against the carburetor body ; @#age 77A

I disagree There is a bit o" slack within the o#ening on the linkage that "its onto thesha"t This means that one could ha$e the carbs #er"ectly balanced but i" the linkage

shi"ted slightly on the throttle #late sha"t the balance would be lost Tightening the nuts

=lightly* will not #re$ent the linkage "rom shi"ting under use

What we disco$ered was that the nuts can and indeed could be tightened securely The

linkage tightens against an indentation on the sha"t, not the carb body, so no binding

should occur !"ter tightening the nuts securely the bit o" slack was gone so that the carblinkage would no longer be able to shi"t on the throttle #late sha"t " course don*t crank

them down too tight or the sha"t may be distorted There is a locktab that bends against

the nut to kee# it "rom $ibrating loose, but "rankly I doubt this has a lot o" signi"icance i"the nut is secured reasonably tight The key is to make sure that there is no #lay in the

linkage on either side o" each carb

When tightening these nuts, hold the ad%oining linkage away "rom the throttle sto# Thatshould #re$ent any distortion o" the throttle sha"t

ne o" the "our carbs #ro$ed to be #roblematic When the nut was tightened it would bind It turned out that one o" the windings on the return s#ring had sli##ed between the

linkage and the carb body nce that was disco$ered it was easily remedied Eee# in

mind, that is how it was assembled by the engine builderF



Weber Carburetors, Tuning. "une, 2009

The ad%ustment #rocedure #retty much "ollowed the $arious books and articles We

started with the mixture screws out turns and the throttle #lates o#ened about a hal"turn "rom where the screw makes contact with the linkage @&at )raden, Weber

Carburetors, recommends starting the mixture screws about a hal" turn out and ad%ust

them out a >uarter turn at a time )ob Tomlinson*s Original Weber Tech Manual

recommends starting about two and a hal" turns out and ad%usting the mixture screw in



+aynes Weber Carburetor Manual recommends #re setting the mixture screws two turns

out I doubt it makes much di""erence which way you go, as long as you end u# in the

right s#otA !"ter the engine was thoroughly warmed, the balance was checked

The two #rimary carbs @connected to the bell crankA were ad%usted "irst, then the two

de#endent carbs

It is interesting how the idle s#eed >uickly smoothes out %ust by getting e$en air "low !

"raction o" a turn o" the sto# screws makes a noticeable di""erence in the idle s#eed ur"inal idle s#eed is around H00 R&'

There was a slight $ariation between the throttle bodies on two o" the carbs The lower

reading was brought u# to the higher with the air screw ad%ustment When the %ob was"inished, all eight throttle bodies were within a needle width o" each other on the air "low

meter

We ha$e had a #ersistent #roblem with one, sometimes two, cylinders not "iring !nin"rared tem#erature gauge is so handy "or >uickly identi"ying a non "iring cylinder It

works much better than a "inger Ty#ically the tem#erature o" the header runs around/00 degrees or more I" one is running around 70 degrees or less, we know it is not

"iring That cylinder*s mixture screw is then o#ened until we hear the R&'s %um#, and

we then know it is "iring The header tem#erature >uickly rises We ha$e "ound that acou#le o" the mixture screws need to be out about a hal" turn or more "arther than the

others nce #ro#erly set, they ha$e been "iring reliably

'aking sure that all cylinders are "iring and getting a good balance between the carbsseems to be the most im#ortant #art o" this #rocess

The mixture screws were ad%usted -irst the idle sto# screw that controls the idle s#eedwas turned in to slow the engine down to around 500 D 300 R&' (ext the mixture

screws were ad%usted by turning them in a >uarter o" a turn at a time until a dro# in the

R&'s could be heard Then it was backed out a >uarter o" a turn and that cylinder wasdone

!"ter the mixture screws are set, the balance is rechecked Then we are done, "or the

moment

Eee#ing a record o" what is changed is hel#"ul We made u# a little chart to document

modi"ications and changes ! "resh sheet is used "or each =session* @We ha$e >uite astack already "illed outA When the #lugs are #ulled, we set them on the chart and sna# a

#icture "or "uture re"erence



Linkage and "etting #ssues. "uly, 2009

!"ter going through the #rocess noted, we thought we had "inally con>uered the Weber*sIt ran well "or se$eral sessions Then Ryan took Cortni "or a ride in late une and

re#orted that the carbs had returned to their old tricks2 s#itting and coughing

While rechecking the balance we made an interesting disco$ery The carbs may be #er"ectly balanced when setting at idle, but when the throttle is #ulled slightly, one bank

went suddenly out o" synch Ryan >uestioned whether there may be some slack in the

bellcrank and linkage that we had not noticed be"ore We care"ully set the idle sto#screws to assure that both banks were matched on the "low meter Then the lock nuts on

the connecting arm on one side were loosened It turns out that there is about an eighth to

a >uarter turn o" slack in the connecting arm, due to the bit o" slack in the bell crank asnoted in a #rior #ost The slack in the bell crank can*t be eliminated, but it can be =tuned*

out by care"ul ad%ustment o" the connecting arms So we did %ust that, tuning out the

slack so both sides #ulled concurrently With Ryan #ushing on the accelerator #edal

holding a steady /00 R&', we con"irmed that the balance was reasonably e>ual withthe "low meter ne cannot %ust #ush on the bellcrank to make this check, since any "orce

a##lied in a di""erent direction than the throttle cable may skew the results

We also re#laced the two return s#rings on the de#endent carbs which had been remo$ed

when we started this #ro%ect The return s#ring connected to the bell crank was remo$ed

since it may ha$e contributed to the une$en "orces being exerted on the bell crank Wedid not want to do anything that would a""ect the geometry o" the linkage



Time "or another dri$e What a di""erence It ran with minimal #o##ing and s#itting

With the carbs running reasonably well, time to "ocus on ="ine tuning* the %ets

!s noted on #re$ious #osts, abo$e .000 R&' there was some hesitation &unch theaccelerator and it took a moment to grab So the "uel %et was made one ste# leaner , "rom

.7 to .0 What a di""erenceF It accelerated without hesitation !bo$e .000 the engine

ran smooth without =#ullling*

+ere is where we ended u# The #icture shows the "inal set u# with the bell crank s#ring

#laced earlier remo$ed The settings are not much di""erent than where we started2 only

the "uel %et is di""erent2

44 I-

Idle2 50

'ain .7!ir 37

8enturi choke tube ./!ux 8enturi 47

Weber Carburetors, $ther #ssues

With nearly a thousand miles on the GT se$eral idiosyncrasies ha$e become a##arent

When starting, we turn the ignition and "uel #um# on and let it run "or ten seconds to

make sure the "uel #ressure is a##ro#riate then one #um# o" the accelerator and a #ush o"the start button It usually starts immediately It amaBes me that the engine starts u# so

easily and idles so >uickly without a choke It coughs and s#its a bit "or the "irst "ew

seconds, then >uickly settles down



! "ew minutes a"ter shutting down a hot engine the "uel in the "loat bowls starts to boil

and dri# into the throttle body and then out onto the intake mani"old This has been

re#orted o"ten on other threads #articularly at wwwClubCobracom It seems to becaused by heat rising "rom inside the engine +eat de"lectors and other remedies seem to

ha$e a limited e""ect

Turning o"" the "uel #um# a hal" mile "rom the destination hel#s The trick is making sure

you don*t shut it o"" too soon, or you may be stalled in tra""ic This has not com#letely

cured the #roblem, but it does hel#, de#ending u#on how much "uel is drained out be"oreshut down

The "uel #ressure regulator was set at / #ounds

We run the engine with wire mesh co$ers on the carbs In street use they do not seem to

make any discernable di""erence The #rotection they a""ord o""sets any loss in

#er"ormance We are not using the black booties, which are more restricti$e We "ound

these $elocity stack "ilters at %ierce Mani!olds They are made s#eci"ically "or the I-carbs (icely made, with two layers o" screening and a $ery "ine mesh inDbetween &art

number 992&'.2(0 &ierce 'ani"olds, Gilroy, C! @40HAH4/D5553

http://www.clubcobra.com/

http://www.clubcobra.com/



Weber Carburetors, Conclusion

This exercise has taught us se$eral lessons2

Jinkage, linkage, linkage

/ The bell crank must ha$e as little slack as #ossible and turn "reely

. The arms connecting the bell crank to the carb must be exactly e>ual in length and

symmetrical in alignment (ylon washers can be added at the heim %oints to make sure

they are symmetrical

4 The nuts on the end o" the throttle sha"ts must be tight so that the connecting arms

can*t sli#, but make sure the throttle sha"t mo$e "reely a"ter the nuts are tight

7 Take o"" the to#s o" the carbs and measure the "loat settings They must be e>ual

5 Clean the idle circuits with com#ressed air a"ter remo$ing the idle %ets and mixturescrews

3 'ake sure the little rings are in #lace 6ach idle %et and each mixture screw has one

H The most critical #hase o" initial tuning is balance o" the carbs I" they are not exactly

balanced the mixture screws won*t much mater <se the idle sto# screws on the two

#rimary carbs to set the balance, then ad%ust the linkage to tune out any slack <se twoidle sto# screws on the two #rimary carbs "or calibration don*t rely on a single sto#

screw Recheck the balance with the engine o"" idle at 000 to /00 R&', #re"erably

while the accelerator is being a##lied rather than #ushing on the bell crank

1 It is amaBing how easily a cylinder will >uit "iring i" the carb ad%ustment is too lean

!n in"rared thermometer sure beats a "inger to check exhaust mani"old tem#s to con"irma cylinder is "iring



0 Checking the #lugs is a good way to hel# con"irm that carbs are e>ually ad%usted, but

ex#ect them to look a bit =rich* i" checked at idle or normal s#eed dri$ing

Eee# a record o" e$ery ad%ustment

/ &atience, #atience, #atience

I cannot imagine using these Webers without the su##ort, ad$ice and wisdom o" those onwwwGT40scom It was a treat to meet Ian Clark and S#yder 'ike in #erson at Road

!merica and thank them "or their in#ut I ho#e I ha$e a chance to thank in #erson the rest

o" you that #ro$ided hel#"ul guidance during this learning #rocess

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gt40webercarburetor.pdf

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