4
4-9
4st 9.9/15/20 2008
When throttle valve is rapidly opened for attempting quick acceleration, the amount of air and vacuous pressure change almost
simultaneously, but there is a tendency that the fuel delays a little because the fuel density is larger than the air, resulting in
making the air-fuel mixture lean instantaneously. To prevent the air-fuel mixture from becoming lean during the acceleration,
acceleration pump is provided to increase the fuel amount during the period to make engine revolution smoother.
The fuel for the acceleration is fed by spring-loaded diaphragm pump that is driven by pump rod and lever assembly. The
diaphragm that is driven returns to its original (unloaded) position by using the spring force in addition to the force produced by
the elasticity of the diaphragm.
As the throttle valve is opened, the rod linking with the valve pushes down the pump rod. The downward movement of the rod
causes the inlet check valve to close, fuel to be pushed into pump discharge passage, outlet check valve to be opened, and then
the fuel is ejected from pump discharge nozzle into the venturi where it is atomized.
When the pump rod moves upward as the throttle valve is closed, the fuel goes through inlet check valve so that the pump well is
filled with the fuel that is used for the next acceleration.
When the pump has discharged the fuel, the check valve closes to prevent further flow of fuel.
a - Air intake
b - Inlet check valve
c - Jet
d - Drain
e - Return spring
f - Diaphragm
g - Diaphragm rod
h - Outlet check valve - Accelerator pump circuit
i - Pump discharge nozzle
4) Acceleration Passage
E_MFS20C_ch04_090105.qxd 09.1.20 5:24 PM ページ 9
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