Fuel Pump Description

Fuel Pump Schematic

fp_1 Cross sectional view of the the fuel pump with major pieces named.


The fuel is pumped to the carburetors by a mechanical diaphragm pump which is mounted on the engine crankcase. The fuel pump is actuated by an eccentric machined into the distributor pinion shaft. The quantity of fuel delivered by the pump is metered automatically in direct proportions to the amount of fuel dispensed by the carburetors.

The fuel pump consists of an upper and lower assembly. The upper assembly accommodates an inlet and outlet valve, and a fuel filter. The lower assembly contains the actuating plunger. Located between both assemblies is a diaphragm and diaphragm spring. The diaphragm is built up of several layers of a fuel-proof material, and is sandwiched between two supporting discs which are riveted to the plunger coupling; the diaphragm also acts as a gasket.

Function Description

The eccentric on the pinion shaft raises the diaphragm actuating plunger. The plunger transmits the pressure to the diaphragm coupling, overcoming the pressure of the plunger return spring but with the support of the diaphragm spring. This forces the fuel contained in the pump to exit through the outlet valve on to the carburetors. When the actuating plunger moves back with further rotation of the eccentric, negative pressure is created in the chamber above the pump diaphragm and fresh fuel enters the pump through the inlet valve. This pumping action repeats with every revolution of the eccentric (once every two revolutions of the crankshaft). The amount of fuel passing through the pump is governed by the amount dispensed by the carburetors; that is, only that amount of fuel can be pumped which is able to pass through the more or less opened float needle valves in the carburetors.
fp_2 Diagram showing how the pump is driven from the distributor pinion shaft.
fp_3 Exploded diagram of the fuel delivery half of the pump.
fp_4 Exploded diagram of the mechanical actuation half of the pump.
Last modified: Wed, 12 Jan 2005


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