4-24. Carburetor (Fig. 4-10)
adjust the carburetor as follows:
a. Adjustment. If troubleshooting indicates that the
carburetor is responsible for faulty engine operation,
8. Main adjusting screw
9. Carburetor float bowl
Throttle cable clamp
10. Flood relief drain cock
Idle speed adjustment screw
11. Choke control cable
Carburetor throttle lever
12. Choke cable clamp
Idle mixture adjustment needle
13. Carburetor choke lever
Carburetor inlet filter
Figure 4-10. Carburetor, showing adjustment points.
(1) Start with engine and allow it to warm to
air admitted to the idling system which functions only at
low speeds. Adjust as follows:
(a) With the engine at idle, turn the idle
(2) Adjust the main adjusting screw (8, fig. 4-
mixture adjustment screw clockwise to cut off air,
10). This adjustment determines the amount of fuel
making the fuel mixture richer, until the engine starts to
available for high-speed operation. Adjust as follows:
run roughly due to excessively rich mixture.
(a) Adjust the throttle control of the pump to open the
carburetor throttle about one-fourth.
counterclockwise to obtain the smoothest idle operation
(b) Turn the main adjusting screw clockwise
to reduce the fuel to the engine until the engine begins
(c) If a vacuum gage is available, use a
to miss due to a lean fuel mixture.
vacuum gage attached to the intake manifold and adjust
the needle for highest engine vacuum.
counterclockwise until the engine runs smoothly without
(4) Adjust the idle speed adjustment screw
Turn the screw clockwise or
(3) Adjust the idle mixture adjustment needle
counterclockwise as necessary to provide 400 to 600
(5, fig. 4-10). This adjustment controls the amount of