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The mechanical fuel pump differs in that it has a vacuum booster
section. The vacuum section is operated by the fuel pump arm;
otherwise, it has nothing to do with the fuel system. During the
suction (or first) stroke, the rotation of the eccentric on the
camshaft puts the pump operating arm into motion, pulling the lever
and diaphragm down against the pressure of the diaphragm spring and
producing suction (vacuum) in the pump chamber. The suction will hold
the outlet valve closed and pull the inlet valve open, causing fuel
to flow through the filter screen and down through the inlet valve of
the pump chamber.
During the return stroke, the diaphragm is forced up by the diaphragm
spring, the inlet valve closes and the outlet valve opens to allow
fuel to flow through the outlet to the carburetor. The operating
lever is hinged to the pump arm, so that it can move down but cannot
be raised by the pump arm. The pump arm spring forces the arm to
follow the cam without moving the lever. The lever can only be moved
upward by the diaphragm spring. This process causes fuel to be
delivered to the carburetor only when the fuel pressure in the outlet
is less than the pressure maintained by the diaphragm spring. This
happens when the passage of fuel from the pump into the carburetor
float chamber is open and the float needle is not seated.
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