SU Fuel Pump won't stop pumping

[seriesmm]

I have a 1957 4 door which has a good, working positive earthed SU fuel pump with the original points system.

As a 'back up' I have a reconditioned 'spare' SU pump of the same type in the boot 'just in case'. It was supplied via the club spares section. It's out of warranty now although its never been used (up until today).

So today I thought I'd just check it works OK - so took the old one off and fitted the 'reconditioned' pump.

Switched the ignition on and it started to 'chatter' but at a much faster speed than the old one. It's pumping fuel through no problem at all but it won't stop pumping - even when the carb float bowl is full it still chatters away albeit at a slightly lower rate. There are no leaks from the float bowl so both float and needle valve are working correctly.

I took the fuel hose off at the outlet (to the carb) on the pump and put my thumb over the end replicating the pressure from a full float bowl in the carb - no matter how hard I pressed on the outlet the pump kept chattering away trying to pump.

Any thoughts on why the pump won't shut off when it should and whether it's possible to adjust the pressure the pump shuts off at?

Many thanks

[svenedin]

It may be an adjustment is required of the points gap but usually there’s an air leak so the pump is drawing air as well as fuel. Check the inlet connection to the line from tank and the actual inlet is tight on the pump body. There’s a fibre washer under there and now it’s wet it may be that the inlet needs to be tightened down.

[geoberni]

If you can't see any fuel leaking, it's probably an air leak in the pump or the check valve letting fuel run back to the tank.
The pump is quite simple, the central armature moves up and down against a light spring pressure, when the diaphragm can no longer move because the carb float chamber has shut off, the spring pressure is unable to push the diaphragm down against the fuel.
The contacts won't make to pull it back up to such more fuel in; it needs some fuel to enter the carb float chamber to create some space for the spring to push the diaphragm down.
See these pages for the Burlin/SU fault diagnosis procedure: https://sucarb.co.uk/technical-electric ... -diagnosis

[seriesmm]

Thank you for your replies, much appreciated.

Drawing air from somewhere sounds a very good suggestion.

I can discount the supply line from the tank as replacing the 'reconditioned' unit with the old, but known good pump and it can pump fuel without issue and shuts off when the float bowl is full.

I've nipped up both inlet and outlet on the 'reconditioned' pump and although covering the outlet with my thumb slows the chatter to a tick it won't stop ticking.

Thanks for the link to the SU fault diagnosis procedure.

[svenedin]

Thank you for your replies, much appreciated.

Drawing air from somewhere sounds a very good suggestion.

I can discount the supply line from the tank as replacing the 'reconditioned' unit with the old, but known good pump and it can pump fuel without issue and shuts off when the float bowl is full.

I've nipped up both inlet and outlet on the 'reconditioned' pump and although covering the outlet with my thumb slows the chatter to a tick it won't stop ticking.

Thanks for the link to the SU fault diagnosis procedure.

You could also tighten the nut for the filter and if that doesn’t help check the screws that secure the diaphragm to the pump body. I have a pump that will tick very slowly maybe every 30 seconds or even 60 seconds it does one tick. I tried to stop it doing that but decided it was of no consequence at all because with the engine running it will use fuel much faster than this infrequent tick. The non-return valve is a simple metal disc and whilst perfectly good I doubt the seal is ever absolutely perfect. You would never normally have the ignition on without the engine running for minutes at a time. Doing so risks damaging the ignition coil.

[seriesmm]

I'm beginning to suspect the non return valve as the issue as I can hear something rattling inside the pump in the area of the inlet/outlets.

I've checked the fuel filter and screws around the diaphragm and they're all tight.

I shall investigate further at the weekend.

Thanks for your suggestions

[svenedin]

I suppose it’s possible the non return valve was not assembled correctly on your reconditioned pump. Have a look at the diagrams on the SU (Burlen) website. The pump is AUA66. The valve (which is a metal disc) needs to be held correctly by a retainer that has springy arms, a type of circlip. Probably not well described but you’ll soon see!

I'd also suggest, even though you have only had the pump connected for a short time that you remove and inspect the filter. It's possible the filter has a hole in the mesh that has allowed dirt through and that has caused the non-return valve to stick slightly open.

Note that the washers in the diagram numbers 31 and 34 are not identical so note the order on disassembly.

https://sucarb.co.uk/su-fuel-pumps-spar ... -8983.html

[gelertd]

Hmmm having done a quick Google regarding SU fuel pumps it would appear that a correctly assembled non return valve assembly is supposed to rattle in its cage when shaken.

I shall still strip it down and see if anything is obviously amiss in there.

Whilst there I’ll check the filter as suggested.

Thank you

[svenedin]

You’re welcome. It’s simple to do and will not take long so a reasonable thing to check.