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Extract from March's MGOC publication - Enjoying MG
by Richard Ladds
Electric fuel pump replacement
When the electric fuel pump in your MGB or A series Midget finally fails, you will have to crawl under the car to repair or replace it. My pump finally stopped working after many years of service. As I had to get the car back on the road immediately I opted for a reconditioned unit, so my task was just to swop the new pump for the old one. In addition to the replacement pump, I also needed four new fibre sealing washers for the fuel inlet and outlet connections.
To check your fuel pump is working you can detach the fuel supply pipe at the carburettors and place the end over a suitable container, then very briefly switch on the ignition. If the pump is working, fuel will flow rapidly from the disconnected line. No fuel flow indicates that the pump is inoperative.
The pump consists of an electro magnetic coil wound inside the pump body. There is also a pair of non return valves, a diaphragm, a spring and a set of points.
These points have an over centre throw system so that when the pump reaches the end of its stroke, the mechanism flips over and closes the points. When the ignition is turned "On" and current flows from the battery to the pump, it passes through the points and energises the coil. This attracts the iron plate at the back of the diaphragm and draws it back. This action closes the outlet valve and opens the inlet valve, allowing fuel to be drawn from the tank to the pump. When the diaphragm is fully drawn in, the centre rod will operate the points and cause them to flip open and cut off the current, the coil will then release the iron plate. As the rod was pulled back it also compressed the spring mounted between the coil and the diaphragm. When the electromagnet is switched off, this spring will return the diaphragm and the operating rod back to the original position. As this happens the inlet valve will close and the outlet will open. Once the pump reaches the end of its stroke the points will close again and withdraw the diaphragm and refill the pump once more.
I dislike messing about with petrol, it is very dangerous and particularly nasty when it pours all over you. Petrol vapour is highly volatile and must be treated with great care, it can be ignited with a single spark. Therefore, avoid using any equipment or performing any task that may cause a spark. I began by disconnecting the batteries on the car, removing the earth clamp first of all. For complete security, I removed both batteries. Removing the driver's side battery, gave me access to the rear of the fuel pump. Another potential source of danger was the mains powered leading lamp; but as the job had to be undertaken at night, I could not dispense with the lamp altogether. So I decided to fix it very securely to avoid any danger from trailing wires or any possibility of a spark. A torch could then provide me with a suitably mobile and safe source of illumination to examine the pump and its various bolts and connections.
Getting at the pump
Jack up the rear of the car and rest the axle securely on purpose built axle stands. Remove the offside road wheel. Now you can see the pump and support bracket bolted onto the body next to the offside battery cage. Remove dirt and surface corrosion from all the connections and fixing bolts, then spray with a liberal coating of WD40.
The worst part of this job is the moment when you disconnect the fuel pipe. If there is a any fuel in the tank, petrol will pour out with considerable force and it will be difficult to stem the flow. You must be ready for this and have a system of blocking the end of the fuel pipe to hand.
There are a number of possibilities, from a suitably sized bolt to a pen or pencil that can be stuffed into the pipe with a rag wrapped around it. Completely by chance, I found the perfect seal for the fuel pipe. On my workbench was a selection of odd bits and pieces, among them were some rubber battery cell cover plugs. These plugs were from the replacement batteries I fitted to my car in the summer. As the batteries are supplied dry charged, these rubber covers are fitted over each cell, presumably to keep out any foreign particles. After the electrolyte is added, the rubber covers are discarded and plastic screw tops put in place.
One of these rubber plugs makes a perfect sealer for the fuel pipe. Just press over the end of the pipe and the fuel is cut off completely without any trace of seepage.
Disconnecting the fuel pipes
Fuel pipes can become old and brittle with age. Being routed along the underside of the car makes them vulnerable to road dirt and corrosion. Treat all fuel pipes and fixings with great care. Always use the correct sized spanner and carefully loosen off all fixings. Undo the fixing brackets and remove all electrical connections and breather or vent pipe connections. Remember to make a careful note of the location of all wires and connections.
Disconnect the inlet and outlet pipe unions. As you undo the fuel supply pipe, be ready with a suitable sealing device and as soon as the pipe comes free, block it off with your finger or thumb and attach the plug. Try to avoid spilling petrol or getting it on your clothes. Be warned, just getting soaked in the stuff can burn your skin. With the fuel supply pipe sealed off, remove the outlet connection to the pipe that takes fuel on to the carburettors. Have a suitable container ready to collect any petrol that may drip out of the pump or run back down the pipe.
Damage or corrosion holes in the fuel pipe will prevent the pump from working properly, making it draw air and preventing the delivery of fuel to the carburettors. Take every opportunity to examine the fuel lines very carefully. Disturbing fuel pipes can often cause fractures, especially if the pipes are old and corroded. If you suspect the condition of the pipes it is a good idea to replace them.
Removing the pump bracket
I found it best to remove the pump and bracket assembly as one unit and then dismantle it off the car. The bracket clamps around the pump over a thick rubber sleeve. Undo the clamp bolt and pull the pump body free.
The pump body is marked "Out" and "Top" to assist you when connecting the fuel pipes and positioning the pump. Fit the replacement pump into the mounting bracket, but do not fully tighten it up until you have the pump and bracket assembly back in place. Offer up the assembly and refit the top and bottom nuts on the mounting bracket. Make sure the pump is in the correct position and that both fuel lines will reach. Make sure you connect these correctly and fit new fibre washers before tightening up all the nuts. Reconnect the power and earth wires and also refit the breather pipes. These breather pipes are important to the functioning of the pump.
When I replaced my pump, I had a nearly full tank of fuel and therefore experienced no problem in getting the fuel to flow back into the pump. However, this is not always the case. If fuel is not coming through from the tank, then do not reconnect until it does. Fuel may not be flowing because the level in the tank is too low, or because the fuel pipe is blocked. Sometimes blowing into the pipe will move debris and restore flow, but take care not to get a mouth full of petrol in the process.
Checking the system
Replace and reconnect both batteries. If everything has gone back together correctly the pump should tick for a few moments until the fuel lines and float chambers are replenished. Once the float chambers are full, the ticking will stop. Finally, examine the fuel pipes, checking all joints and pump connections to ensure that petrol is not leaking anywhere. The engine should now start without any further problems.
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