Fuel / Exhaust
4.35
Fuel Pump Operation
The pump supplies fuel to carburetors. On three cylinder models two fuel pumps supply fuel to each of the end
carburetors and commonly to the center cylinder.
When the piston is on the upstroke
crankcase pressure is low, the
diaphragm in the fuel chamber moves
outward toward the chamber cover. This
causes a negative (low) pressure in the
fuel chamber . The check valve on the
fuel supply side of the chamber opens,
allowing fuel to enter. This vacuum
causes the outlet check valve to close so
fuel cannot return from the carburetor.
See Ill. 1.
When the piston is on its downstroke,
the pressure from the crankcase
becomes positive causing the
diaphragm to move in the opposite
direction, reversing the pressure in the
pump chamber. This causes the inlet
check valve to close and the outlet valve
to open, delivering pressurized fuel to
the carburetor fuel chambers. see Ill. 2.
Fuel in
Ill. 1 Piston on upstroke
Negative
(low)
crankcase
pressure
Inlet check
valve open
Fuel return
to tank
Outlet check
valve closed
Diaphragm
Ill. 2 Piston on downstroke
Positive
(high)
crankcase
pressure
Fuel return
to tank
Inlet check
valve closed
Outlet check
valve open
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