A.C. Mechanical Petrol Pump

 [The construction and operation dealt with the most common type of carburettor, the side draft Zenith. However, the carburettor is only of use if it gets a supply of Petrol. This article was found in an undated “Practical Home Handyman”, found by Richard Butterworth for 30p at the St Mellion Cherry Feast White Elephant stall! ] 

In nearly all modern motor vehicles the petrol tank is situated well below the level of the carburettor. Consequently, as the petrol will not flow to the float chamber some means must be provided to deliver fuel to the carburettor. In the past, it was sometimes the practice to locate the tank under the scuttle and fed the carburettor by gravity. But now it has become the practice to position the tank at the rear of the car. This very often involves lifting the petrol a matter of 2 ft. to the carburettor float chamber. Suction pumps which utilized engine induction pressure for the operation of a vacuum for the delivery of fuel from tank to carburettor were once very popular, but these are now obsolete having been replaced by pumps with positive drives. These diaphragm type pumps have now become standardized fitting on all vehicles. To how these pumps work we will describe the action of a typical fully mechanical pump.

mech fuel pump

The pump is usually bolted to the crankcase of the engine and operated directly by an eccentric on the camshaft, or by push rod. As the camshaft rotates, the eccentric lifts the rocker-arm which pulls the connecting rod, together with the diaphragm, downward against the pressure of the return spring, thus creating a vacuum in the pump chamber. Fuel from the tank is then sucked through the inlet connection, into the sediment chamber, through the gauze filter and inlet valve into the pump chamber. On the return stroke the pressure of the return spring pushes the diaphragm upward forcing fuel from the pump chamber through the outlet or delivery valve and outlet connection to the carburettor. 

When the correct level in the carburettor float chamber is reached, the needle valve will close, thus creating a back pressure in the pump chamber. This pressure will hold the diaphragm downward against the return spring and it will remain in this position until the carburettor requires more fuel and the float chamber needle valve opens.

When the fuel pump is subjected to a back pressure the diaphragm connecting rod forces the interior, pivoted portion or lever of the two-piece rocker arm to the bottom of its stroke. As the outer part of the rocker arm, which is in direct contact with the eccentric cam, is also secured to the same pivot centre as the lever, the rocker arm will cease to operate the lever until the diaphragm is returned to its initial position.  The small spring at the rocker arm shoulder is intended to keep the rocker arm in constant contact with the eccentric, to eliminate noise.  

The gauze filter should be regularly cleaned. It can be taken out when the cover of the sediment chamber is removed. Make certain that the cork gasket is properly seated and that the fibre washer is under the head of the screw when re-assembling, so that there is no air leak into the sediment chamber. Any deposit in the sediment chamber can usually be drained off by removing the drain plug.  

Seepage of fuel at the edge of the diaphragm can generally be cured by tightening up the body screws. A continual fuel leakage from the drain hole in the body casting usually indicates a punctured diaphragm. Should this happen it is necessary to change the diaphragm. This can be achieved by carefully separating the upper and lower castings once the pump has been removed from the engine block. Simply remove the body screws and separate the two halves, taking care when refitting to tighten them sequentially but not excessively.


First published in Seven Focus Sept 2007 pp19-20


See also:

The AC Fuel Pump