Proper operation of the fuel system is essential for smooth engine performance, reliable starting and correct fuel delivery under all driving conditions. The fuel system consists primarily of the fuel tank, fuel lines, fuel pump, vacuum booster pump and carburetor. Faults in any of these components may lead to poor engine performance, starting difficulties or excessive fuel consumption.
When diagnosing fuel system problems, inspection should begin with the fuel tank and supply lines, then continue through the fuel pump and carburetor system.
Fuel Tank and Lines
Water, dirt and other contaminants that accumulate in the fuel tank can cause carburetor malfunction or fuel pump problems.
One of the most common sources of water contamination is condensation, which forms when moisture in the air inside the fuel tank condenses on the cooler metal tank walls.
If excessive sediment is found in the fuel pump sediment bowl, the fuel tank should be removed and flushed thoroughly. The fuel line between the tank and pump should also be blown out to remove debris and contamination.
Keeping the tank and fuel lines clean ensures reliable fuel delivery and prevents carburetor and pump damage.
Fuel Pump and Vacuum Booster
Incorrect fuel pump pressure or low fuel pump capacity are the most common pump-related faults affecting engine performance.
Low fuel pressure can cause a lean fuel mixture at high engine speeds, while excessive pressure may lead to carburetor flooding and increased fuel consumption.
Low pump capacity can also cause fuel starvation at higher speeds, resulting in loss of engine power.
Fuel Pump Pressure Test
Fuel pump tests should be performed with the pump installed on the engine.
-
Disconnect the fuel line at the carburetor.
-
Install a pressure gauge and T-fitting with a petcock between the gauge and carburetor inlet fitting.
-
Vent the system by briefly opening the petcock before taking a pressure reading.
-
Run the engine at the specified speed and observe the pressure gauge.
After the reading stabilizes, fuel pressure should fall within the specified range.
Fuel Pump Capacity Test
The capacity test should only be performed after the pressure test confirms the pump is operating within specifications.
-
Operate the engine at the specified rpm.
-
Open the petcock and allow fuel to flow into a suitable container.
-
Measure the time required to deliver one pint of fuel.
If the fuel delivery time is outside specifications, remove the fuel filter and repeat the test.
If capacity is within specifications with the filter removed, the fuel filter is restricted and should be replaced.
If capacity is still outside specifications with the filter removed, the fuel pump is defective.
Vacuum Booster Test
The vacuum booster portion of the pump is used to assist vacuum-operated components such as the windshield wipers.
To test the vacuum booster:
-
Connect a vacuum gauge to the windshield wiper connection on the fuel pump.
-
Disconnect the pump-to-manifold line at the intake manifold and plug the manifold connection.
-
Run the engine at the specified rpm and observe the vacuum gauge.
The vacuum reading should fall within specified limits.
Fuel Pump Trouble Diagnosis
Common fuel pump problems and causes include:
Low Fuel Pump Pressure
-
Ruptured, stretched or leaking diaphragm
-
Weak spring
-
Worn rocker arm
-
Excessive clearance between rocker arm, vacuum link and fuel pump link
-
Loose or cracked fuel line fittings
-
Dirt in the fuel tank or fuel lines
-
Restricted fuel tank vent
High Fuel Pump Pressure
-
Spring too strong or incorrect replacement spring
Fuel Pump Leaks Fuel
-
Loose main body retaining screws
-
Defective diaphragm
-
Loose fittings
-
Stripped threads on fittings
-
Cracked pump body
Fuel Pump Leaks Oil
-
Defective pull rod oil seal
-
Loose fuel pump mounting bolts
-
Defective mounting gasket
Fuel Pump Noise
-
Loose mounting bolts
-
Worn rocker arm
-
Weak or broken rocker arm spring
Loss of Booster Pump Vacuum
-
Loose vacuum pump cover screws
-
Valves not seating correctly
-
Weak spring
-
Defective diaphragm
Carburetor Trouble Diagnosis
Most carburetor problems are caused by dirt accumulation in fuel or air passages, incorrect idle adjustments or improper fuel level settings.
Accelerating Pump Test
To test accelerating pump operation:
-
Remove the air cleaner.
-
Open the throttle plate.
-
Observe the fuel discharge from the accelerating pump discharge nozzle.
If the system is operating correctly, a quick steady stream of fuel should discharge from the nozzle.
Carburetor Trouble Guide
Hard Starting (Hot or Cold)
Possible causes include:
-
Binding or broken choke linkage
-
Improper starting procedure flooding the engine
-
Incorrect carburetor fuel level
-
Incorrect idle adjustments
-
Sticking or incorrectly seating fuel inlet needle
-
Incorrect fuel pump pressure
-
Improper carburetor gasket or spacer combination
-
Clogged fuel filter
Poor Idle
-
Incorrect idle mixture adjustment
-
Worn or damaged idle adjustment needles
-
Restricted idle air bleeds
-
Restricted idle fuel passages
-
Idle discharge holes restricted or misaligned with throttle plate
Poor Acceleration
-
Defective accelerating pump diaphragm
-
Incorrect pump stroke adjustment
-
Fuel inlet ball check not seating
-
Low fuel level or incorrect float setting
-
Restriction in accelerating pump discharge passage
-
Discharge ball check not seating properly
-
Air leaks between carburetor and spacer or at throttle shaft
-
Clogged fuel filter
Flooding or Fuel Leakage
-
Cracked carburetor body or fuel bowl
-
Defective bowl or main body gasket
-
High fuel level or float setting
-
Fuel inlet needle not seating properly
-
Ruptured accelerating pump diaphragm
-
Excessive fuel pump pressure
Poor Performance Caused by Rich Mixture
-
Excessive dirt in air cleaner
-
High fuel level or float setting
-
Worn or improperly seating needle and seat
-
Power valve leakage
-
Restricted air bleeds
-
Worn or damaged main jet
-
Fuel pump pressure excessive
Poor Performance Caused by Lean Mixture
-
Low fuel level or float setting
-
Restriction in main fuel passage
-
Sticking fuel inlet needle
-
Low fuel pump pressure
-
Clogged fuel filter
Proper inspection and testing of the fuel system will ensure correct diagnosis and prevent unnecessary replacement of components.
Safety, Copyright and Source Notice
Safety Disclaimer
Working on motor vehicles involves mechanical, electrical and chemical hazards. Always ensure the vehicle is safely supported, the engine is switched off when appropriate, and suitable safety equipment such as gloves and eye protection are used. If you are unsure about any repair or maintenance procedure, consult a qualified mechanic before attempting the work.
Source
Information compiled from period factory service literature.
Copyright © Car Manuals Direct. All rights reserved.
