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 feed the carburettor by gravity, and this was especially popular with commercial vehicles. But now it has become the practice to position the tank at the rear of private cars, and at the side of commercial vehicles. This very often involves lifting the petrol a matter of 2ft. 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vacuum for the delivery of fuel from tank to carburettor were once very popular, but these are now obsolescent; having been replaced by pumps with positive drives, electrically or mechanically operated.
These pumps (of the diaphragm type) have now become the standardized fitting on all vehicles. To see how these pumps work we will describe the action of a typical electrically operated pump of popular design, and also that of another pump operated mechanically by the engine. If the working of these two typical pumps is understood, no difficulty should be experienced in understanding the various other makes that may be encountered.
S.U. Electric Petrol Pump — This pump is capable of a suction lift of 4ft. And will maintain a continual supply of petrol to the carburettor entirely automatically. It is electrically operated and is usually in circuit with the ignition switch, consequently it will only work when this switch is in the ON position.
The action of the pump is as follows:
When the current is switched off and the pump is at rest, the outer rocker of the contact breaker is in the outer position and its tungsten point is in contact with the tungsten poin on the spring blade. When the current is earthed, it passes from the current terminal through the coil, back to the spring blade, and through the tungsten contacts to earth. This completes the circuit and energizes the magnet coil, thus attracting the iron armature core, which comes forward on the rollers bringing the diaphragm with it. This fnovement of the diaphragm creates a temporary vacuum which draws petrol from the petrol tank, through the inlet valve into the fuel chamber of the pump body. When the armature has advanced nearly to the end of its stroke, the contact breaker rod, which moves with it, operates a ‘throw over’ mechanism on the contact breaker, separating the points and breaking the circuit. The delivery spring then pushes the armature and diaphragm back, forcing petrol through the outlet or delivery valve to the carburettor. As soon as the armature gets near the end of this stroke the ‘throw over’ mechanism again operates, the breaker points again make contact, and the cycle of operations is repeated. The quantity of fuel delivered to the carburettor float chambers is governed by the float chamber needle valve, which, when closed, sets up a back pressure exceeding the strength of the fuel pump diaphragm return spring.
If trouble is experienced, and the pump ceases to function, first make sure that current is available at the pump terminal by shorting the wire to earth and seeing if it sparks. If there is no current the trouble is obviously not the pump but in the electrical system. But if current is available and the pump still ceases to operate it is probably due to dirty contact points. These should be cleaned by dressing with a breaker-point dressing slip, after which the pump will probably begin to operate again. If this does not cure the trouble, make sure that the breaker points are making contact. If they are not, it means that the contact breaker rod wants adjusting. As this is a somewhat complicated job necessitating the dismantling of the pump, it is advisable to obtain the assistance of a competent mechanic before effecting re-adjustment.
On the other hand, if the pump works noisily an air leak on the suction side may be suspected. Obviously, if the petrol tank is empty the pump will be working without delivering petrol, so do not overlook this point. Also do not neglect cleaning the filter regularly. Dirt underneath the suction or delivery valves will cause the pump to continue noisily working with little or no delivery of petrol. Each valve can be cleaned easily by unscrewing the top union from the body and lifting out the valve cage.
A.C. Mechanical Petrol Pump
This fully mechanical pump is particularly popular, and its manufacturers maintain a very fine service organization.
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 iv’co 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 advisable to get the pump exchanged for a serviceable unit.