(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,
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.
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