Post
by SHyslop » Sun Sep 01, 2024 3:23 pm
Dear Thomas
This is just drawn from my own experiences but hopefully something in it may be of use to you.
I've owned / still own several cars with their original fuel systems and others with modifications. The AC mechanical pump can have a rather unfairly bad reputation for a variety of reasons. Replacing it with an SU pump can solve some of these, but so would finding and solving the original problem. Having an electric pump in the circuit can be of use as long as exactly what it is doing is fully appreciated.
Now for some details. Starting at the back of the car (the fuel tank), if the pickup to the fuel and line and fuel line itself is blocked , or can become blocked, it won't matter what pump you have in the circuit. Establishing the fuel line to the pump is clear is the first step to good fuel delivery.
Next, be aware there are two types of SU pump. There are pumps which push and pumps which pull. A puller is at the front and that's the type you have, a pusher at the back. I've established to my own satisfaction, with 4 cars, that the 6 cylinder cars with a Solex carburettor, need a pusher but the four cylinder cars, with either an SU or a Solex, will be adequately served by a puller on the bulkhead.
The AC fuel pump worked reliably for many years and has provided petrol for what must have been millions of miles across the world. The failings in it today can be caused by 1) lack of air tightness in the top cover and/or valves, 2) wear in the actuating arm,3) blocked filer and dirt in the petrol chamber,4) worn diaphragm.
Fixing these will return a pump to optimum efficiency. One item which is not covered by most books is that overtightening of the cover screw will crack the casting it screws into and this will result in a lack of suction and can prove very difficult to identify. Also, the quality of new made reproduction valves (the ones in a metal cage) is, in my view, poor and when original AC valves are used, they usually prove to be satisfactory.
To the best of my knowledge, even for cars that have covered more miles than most Rovers ever would- (I'm thinking of Austin 12 taxis, for example) (then there's Daimlers,Lanchesters,RM Rileys...similar mileages/use), no secondary firms made reproduction actuating arm in the 30s-50s. Either replacement pumps were so cheap that replacing the arm was never thought of ; and/or the arms wore so little that their replacement wasn't considered necessary.
A variety of new parts ,including filters,valves, screws and washers are all available from Flexolite.
Worn diaphragms of new material supposed E10 resistant are now made.
The pump should be able now to suck, from dry, petrol to the carburettor in under about 20 strokes of the hand priming lever, less of course if there is petrol in the line. If the car is left unused for lengthy periods then an electric pump placed before the mechanical pump would remove the need to use the hand primer. However, the pump will only pump the petrol to the petrol chamber in the pump and no further. I have a 10 with a separate low pressure electric pump controlled by a separate switch so it can be used when needed and switched off thereafter.
The valves of the mechanical pump can be removed and an electric pump placed in the circuit thereby delivering fuel direct from the tank to the carburettor.
Another method is to have an electric pump fitted with T junctions enabling excess fuel to go back round 'the circuit', the bypass you you
have mentioned in your post. I have a 1935 Daimler 15 with this arrangement and it seems to be satisfactory.
In short, there are various ways of making it all work but the main thing is to have whichever arrangement working well. If you wanted an extra "insurance policy", you could fit a T piece after the electric pump, a two way tap, then another T piece after into the line to the carburettor and that would enable the electric pump to work in place of the mechanical pump in the event of the mechanical pump failing,
Out of interest, I thought the 47 cars had an electric pump fitted rather than a mechanical one. I have a 47 12 exported to Jersey and it is so equipped. I wonder if that means your car was destined for the Home market originally? Just curious!
