CARBURETED FUEL SYSTEM

General layout of fuel supply system in Petrol Engine

Mechanical Fuel Pump

Diagnosing troubles in Carbureted fuel Systems

• Visual inspection and• Relieving fuel line pressure.

Visual inspection

• With engine off, check the engine compartment for possible fuel or air leaks.

• A fuel odour indicates a leak.• Examine all fuel line connections from from

fuel pump to spark plug.• An accumulation of dirt around a connection

could mean a fuel leak.• Install a new seals as necessary and replace any

defective hoses.

Relieving fuel line pressure

• The fuel pressure must be relieved before disconnecting a fuel line.

• Many fuel systems use a Schrader valve, similar to a tire valve, as pressure relief valve.

• To relieve the fuel line pressure, remove the cap from the valve.

• Wrap towels around the fittings to absorb the fuel.

Mechanical fuel pump troubles

1) Insufficient Fuel Delievery• Broken, Worn out or Cracked diaghram,• Improperly operating fuel pump valves,• Broken or damaged fuel pump rocker arm,• Clogged fuel pump filter.

Mechanical Fuel Pump

2)Excessive fuel pump pressure• High fuel pump pressure causes delivery of

too much fuel to the carburetor. The excessive presssure forces the needle valve off its seat, raising the fuel level too high in the float bowl.

3) Fuel pump noises• Noisy pump is usually a result of worn or

broken parts in pump. Worn or broken rocker arm pin, Broken diaghram spring.

• A loud noise can be felt by touching the pump with your hand.

Testing mechanical fuel pumps

• Tests are made for:– pressure, – volume and – vacuum.

• With the engine idling, the gauge shows fuel pump pressure. Typical mechanical fuel pump pressure is from 4 to 7 psi ( 27.6 to 48.3 kPa).

• Place the end of the flexible hose in a graduated container.

• Open the hose restrictor for 30 seconds.• With the engine idling, a good mechanical fuel

pump delivers one pint ( 0.47 litre) or more.

• Stop the engine and connect a vacuum gauge to the fuel pump inlet or vacuum side.

• Then start the engine and let it idle on the fuel in the float bowl.

• A minimum vacuum reading is usually 10 inches ( 250 mm) mercury at idle.

Carburetor troubles

1) Excessive fuel consumption can result from:• High float level,• Worn jet or nozzle,• Leaky carburetor,• High fuel pump pressure,• Improperly adjusted choke,• Faulty or improperly adjusted throttle position.

2) Lack of engine power, acceleration or high speed performance:• Dirt or gum clogging the fuel nozzle or jets,• Low float level,• Dirty air filter,• Stuck or non operating choke,• Air leaks into intake manifold,• Throttle valve not open fully.

3) Failure of the engine to start:• No fuel in tank or carburetor,• Air leaks into mechanical fuel pump due to poor

connections,• Clogged fuel lines,• Clogged carburetor jet.

4) Slow engine warm up due to defective choke.

5) Smoky, black exhaust gas is due to a very rich mixture.

Carbureted fuel system service

Carbureted fuel filter service:• Fuel filter requires no service other than periodic

maintenance.• On some cars, the filter is part of the fuel pump. On

others filter attaches to the carburetor.• Replace this filter by unclamping and deteching the

hose.• Then remove and discard the filter.• Be carful not to over tighten or cross thread the new

filter when installing it. This can strip threads and cause a leak.

Mechanical fuel pump service:• Most mechanical fuel pumps that fail are

replaced.• Wipe off any dirt or grease so it will not get into

the engine.• Disconnect the fuel lines.• Remove the attaching nuts or bolts and lift off

pump.• If it sticks, work it gently from side to side.

• Make sure fuel line connections are clean and in good condition.

• Place a new gasket on the stands of the fuel pump mounting pad.

• Mounting surface of the engine should be clean.• Insert the rocker arm of fuel pump into the opening.• Make sure that arm goes on the proper side of the

camshaft.• Install the attaching nuts or bolts.• Tighten the fuel lines, start the engine, and check for

leaks.

Carburetor removal:• First disconnect the ground cable from the

battery ground.

1) Take off the air cleaner.

2) Disconect the throttle and choke linkage.

3) Disconnect the fuel lines from carburetor, and nut or bolts.

Use two wrenches to avoid damaging tubes.

5) Take off the carburetor.

6) Hold the carburetor level to avoid spilling fuel from the float bowl. Pour the fluid into a safety container.

7) Put the carburetor in a clean place where dirt cannot get into it.

8) Cover the hole in the intake manifold with masking tape.

Carburetor cleaning and rebuilding:

1) Empty any gasoline remaining in the carburetor. Place the parts in small pan.

2) Soak the metal castings and parts in carburetor cleaner.

3) After soaking for specified time, remove the parts.

4) Blow out all jets and passage in castings with compressed air.

5) Replace any damaged or worn parts.

6) Shake hollw floats to determine if they leak and have liquid inside.

7) Weight floats.

8) Check choke housing for deposits and corrosion.

9) Inspect the venturi nozzle, replace it if damaged.

10) Install all of the new gasket.

11) Before completing final assembly of the carburetor, set the float level. Improper operation of the float system affects fuel delivery to all other systems.

12) Install a new fuel filter. Fill the float ball with fuel before installing the carburetor.

Carburetor installation:• Remove tape placed over the intake manifold

openings.• Check that mounting surface is clean and install the

carburetor.• Use a new gasket to assure a tight seal.• Installation is reverse of removal.• Start the engine and check for air, fuel, and

vacuum leaks.• Road test the car to check for proper operation and

drivability.