Fuel Pump Maintenance

The SU electric fuel pump used in the Magnette is one of those components where “no news is good news”! In a very real sense it is the heart of the car. Its tick is reminiscent of a heartbeat and its pulse is essential to the car’s function. It seems to soldier on for a long time requiring little regular maintenance other than cleaning the points with a piece of rough card and washing the filter in clean fuel. However, it prefers regular use and it seems that once a car is laid up for a while, the pump loses the work ethic and starts to cause problems. A Magnette owner observed to be stopping at regular intervals to run to the rear of the car and work fervently in the boot is probably administering a smart tap on the fuel pump with a spanner to encourage it to resume operations! At least with the pump safely tucked in the boot, the Magnette owner does not have the risks of MGA and MGB owners, whose pump is under the car exposed to wet and dirt.

 This sort of unreliability problem is often caused by heavier corrosion of the points and can be rectified by drawing a strip of emery paper or a similar mild abrasive through them. (If the problem arises on the road and you have a lady passenger, ask if she has an emery board in her handbag. Better still, keep one in the glovebox). It is also worth checking that the pump has a good earth connection and is receiving all the current it can get.

Some older pumps are polarity-sensitive, so you will get problems if you change to negative earth without modifying the pump accordingly. Burlen Services in Salisbury will supply replacement parts and advise on what you need.

With the ignition on but the engine not running, it is normal for the pump to tick occasionally as pressure in the system dissipates. However, if it ticks more often than once every ten seconds, suspect a leak somewhere in the system and check for problems with the needle valve in the carburettor float chambers. The latter problem will be evidenced by loss of fuel through the float chamber overflow pipes.

Over longer periods and extended mileage, there is likely to be a need for more fundamental work. Like ignition points, the contacts in the pump will eventually become so corroded or pitted as to be unserviceable. The diaphragm may also perish and lose its elasticity so that the pump is weakened. As it is necessary to dismantle the pump to renew the points, it is logical to renew points and diaphragm together. The workshop manual describes the procedure for stripping and rebuilding the pump. Cleanliness is vital to avoid contamination and blockage. Burlen Services can supply a rebuild kit.

The most frequent error committed by the owner mechanic is to omit the pre-tensioning of the diaphragm described in Section B5. Unless this procedure is carefully followed, the “throw-over” reciprocating action of the points will not work properly and the pump will work only intermittently if at all. As the manual suggests, with a new diaphragm, quite a bit of upward pressure is needed to achieve the right degree of pre-tension, so that the pump throws over with a strong, decisive click. It is also important to tension the diaphragm when tightening the six main screws that hold the body parts together. This is achieved by inserting a suitable wedge under the trunnion that is attached on top of the pump to the threaded central rod connected to the diaphragm. This pulls the diaphragm upwards to the full extent of its working stretch.

A pump that is working but not generating enough pressure will also cause performance problems, so if you suspect fuel starvation is the cause of poor acceleration or loss of top speed, check the pump’s performance. This can be done with the pump on the car by disconnecting the fuel pipe where it connects to the carburettors and connecting another length of pipe to it so that you can divert the flow to a suitable container. It should deliver 1 Imperial pint (= 0.57 litre) per minute. The pump is self-priming, so should be able quickly to draw fuel from the tank and start delivering it to the carburettors.

If you remove the pump from the car, use something like an elastic band or coloured tape to mark the pipe that connects to the pump output union. This is the one right at the top when the pump is correctly mounted. Connecting the pipes the wrong way round can cause hours of fun tracing the fault.

Fuel and sparks are a great combination in the engine, but if they come together in the boot of your car, it’s not such a great idea. When working on the pump, make sure the ignition is off whenever the fuel connections are open or when there is inevitable fuel spillage after dismantling. Don’t switch on to test the pump until you are sure no dangerous sparks can happen.

See the separate Fault Finding article if you need to trace a fuel pump problem.

Some owners decide to opt out of the potential problems with SU pumps and fit alternatives. See the Modifications section for details.

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