Accelerator Pump Circuit.

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Figure 4-32. Vacuum-Operated Metering Rod.

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TM-9-8000 Principles of Automotive Vehicles Manual

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Figure 4-35. Diaphragm Accelerator Pump.

TM 9-8000

(1) The pump piston Is pushed down In the

418. Accelerator Pump Circuit.

pump chamber as the throttle valve is opened, forcing

gasoline through the outlet passageway.

a. Purpose. When the throttle valve Is suddenly

opened, there is a corresponding sudden increase In the

(2) At the same moment, the inlet check ball will

speed of the airflow through the carburetor. Because the

seat keeping gasoline from being pumped back into the

air is lighter than the gasoline, it will accelerate quicker,

float bowl.

causing a very lean mixture to reach the engine for a

brief period. This would result in a severe lag In engine

(3) The discharge check needle will be forced off

performance if not for the accelerator pump circuit. its

its seat, allowing gasoline to pass to the pump discharge

job is to inject a measured charge of gasoline into the

nozzle, where it will be discharged into the throttle of the

carburetor throat whenever the throttle valve is opened.

carburetor.

b. Operation. (Fig. 4-34). The accelerator pump

(4) The pump piston Is raised in the pump

circuit consists of a pump that is operated by linkage

chamber when the throttle valve Is closed, causing the

directly from the throttle valve. There are passageways

discharge check needle to seat, blocking the outlet

that connect the pump to the float bowl and pump

discharge nozzle. There are two check valves in the

system to control the direction of gasoline flow.

Operation is as follows:

Figure 4-34. Accelerator Pump Circuit.

Change 1 4-21