Electric Fuel Pump: Fault Diagnosis
Suspected Fuel Feed Failure
Disconnect the fuel line at the carburetter and check for flow. If normal, examine the carburetter for obstructed float-chamber needle seating or gummed needle.
If normal initially, but diminishing rapidly and accompanied by slow pump operation, check for correct tank venting by removing the filler cap. In adequate venting causes a slow power stroke, with resultant excessive burning of contact points.
If a reduced flow is accompanied by slow operation of the pump, check for any restriction on the inlet side of the pump, such as a clogged filter, which should be removed and cleaned. In the case of a reduced flow with rapid operation of the pump, check for an air leak on the suction side, dirt under the valves, or fault valve sealing washers.
If there is no flow, check for:
(a) Electrical Supply
Disconnect the lead from the terminal and test for an electrical supply.
(b) Faulty Contact Points
If electrical supply is satisfactory the bakelite cover should be removed to check that the tungsten points are in contact. The lead should then be replaced on the terminal and the top contact plate connected briefly to a good earth. If the pump then performs a stroke the fault is due to dirt, corrosion or maladjustment of the tungsten points.
(c) Obstructed Pipeline Between Fuel Tank and Pump
The inlet pipe should be disconnected; if the pump then operates, the trouble is due to a restriction in the pipeline between the pump and the tank. This may be cleared by the use of compressed air after removing the fuel tank filler cap. It should be noted, however, that compressed air should not be passed through the pump, as this will cause serious damage to the valves.
(d) Faulty Diaphragm Action
If the previous operations fail to locate the trouble, stiffening of the diaphragm fabric or abnormal friction in the rocker throw-over mechanism is then to be suspected. To remedy these faults, the coil housing should be removed and the diaphragm flexed a few times. Prior to reassembly, it is advisable to apply a little thin oil to the throw-over spring spindles at a point where they pivot in the brass rockers. The diaphragm/armature assembly should then be assembled and set.
If the pump is noisy in operation, an air leak at one or other of the suction lines may be the cause. Such a leak may be checked by disconnecting the fuel pipe from the carburetter and allowing the pump to discharge into a suitable container with the end of the pipe submerged. The continuous emission of bubbles at this point will confirm the existence of an air leak. The fault should be rectified by carrying out the following procedure:
(a) Check that all connections from the fuel tank to the pump are in good order.
(b) Check that the inlet union is tight and that the sealing O rings are not damaged.
(c) Check that all the coil housing securing screws are well and evenly tightened. Air leaks on the suction side cause rapid operation of the pump and are the most frequent cause of premature failure.
Pump Operates Without Delivering Fuel
If the pump operates without delivery fuel the most likely causes are:
(a) A serious air leak on the suction side. Remedy using the instructions above.
(b) Foreign matter lodged under one of the valves, particularly under the inlet valve. Remove the valves for cleaning, taking care that the Melinex material of the valve is not scratched or damaged during this operation.