3-04
Fuel System
3-04
SYMPTOM
PROBABLE CAUSE
Inconsistent Engine
Incorrect throttle linkage
Excessive looseness of throttle
Idle Speed
adjustment to carburetor.
shaft in bores of throttle body
Governor not adjusted properly
Incorrectly installed throttle
or faulty.
plates.
Binding or sticking throttle
Sticking fuel inlet needle.
linkage.
Defective spark valve or gasket
Sticking carburetor throttle shaft.
(manual choke carburetor)
Surging Above Idle
Clogged main jets.
Clogged fuel filter or fuel
Speed
Improper size main jets.
pump filter screen.
Low fuel level or float setting.
Distributor vacuum passage
Low fuel pump pressure or
Clogged.
volume.
Defective spark valve or gasket
Reduced Power Output
Float setting too high or
Restriction in main fuel
too low.
passages.
Fuel pump pressure too high
Excessive dirt in air cleaner.
or too low.
Throttle plate not fully open.
Improper size or obstructed
Faulty choke operation.
main jets.
Improper throttle linkage or
Restricted air bleeds.
Governor adjustment.
2.
Connect a pressure gauge, a restrictor and a flexible hose
PRESSURE AND CAPACITY (VOLUME) TESTING
(Figure 3) between the fuel filter and the carburetor. NOTE:
To determine that the fuel pump is in satisfactory operating
condition, tests for both fuel pump pressure and fuel pump capacity
Inside diameter of smallest passage in test now circuit must
(volume) should be performed.
not be smaller than .220.
The tests are performed with the fuel pump installed on the
engine and the engine at normal operating temperature at idle speed.
3.
Position the flexible fuel outlet hose and the restrictor so the fuel
Before the tests, make sure the replaceable fuel filter has
can be discharged into a suitable graduated container (Figure 3).
been changed within the recommended maintenance interval.
4.
Before taking a pressure reading, operate the engine at the
When in doubt, install a new filter.
specified idle rpm and vent the system into the container by
opening the hose restrictor momentarily 5. Close the hose
restrictor. allow the pressure to stabilize, and note the reading
(Refer to the Specifications in Part 8)
Pressure Tests
If the pump pressure is not within specifications. and the fuel
Refer to the fuel pump specification and note the fuel pump
lines and filter are in satisfactory condition. the pump should be
pressure and capacity (Volume) design tolerances.
replaced.
1.
Remove the air cleaner assembly. Disconnect the fuel inlet line
If the pump pressure is within specifications, perform the tests
or the fuel filter at the carburetor. Use care to prevent
for fuel capacity (volume).
combustion due to fuel spillage.
FIG. 3. Typical Fuel Pump Pressure and Capacity Test Equipment
