Odd fuel pump thing

[alex_holden]

I've rebuilt two fuel pumps now. The first one I rebuilt with new old stock points and diaphragm. The diaphragm was sealed in a cardboard box dated sometime before I was born. That pump works perfectly.

The second pump I rebuilt with brand new points and diaphragm. It ran OK for about 200 miles, then started randomly cutting out. Once it started doing this, tapping it would make it pump once per tap. Leave it overnight and it ran OK again for a while. Close examination revealed that the points were throwing over into the 'off' position, then not returning far enough to throw over into the 'on' position again. At first I blamed the points because they don't look to be as well made as the old ones, but I changed them for an old set taken from a different pump (used but not totally worn out) and it still got stuck. Eventually I discovered that the only thing that cured the problem was switching back to the original diaphragm (which looks a bit tatty but isn't leaking). I suspect the actual problem has to do with the tolerances of the metal flange part of the diaphragm. It seemed to be trapping the plastic spacer tighter than the original, increasing the force required to move the diaphragm.

[paulhumphries]

Great !
I've a new diaphragm and points that I keep with me for "emergencies".
I suppose now I'm going to have to actually going to try them on a spare pump as I suspect they will be as bad as they ones you bought
I suppose it's a case of if the original is working OK leave it alone.

Paul Humphries

[Orkney]

Have a feeling that read on here a while back someone mentioning that the new diapraghms could be quite inferior to older ones depending on where you bought them?
Original symptoms sound a bit like teh fuel tank not being able to breathe - thus forming a vacuum requiring greater suction - not changed your fuel filler cap lately?

[alex_holden]

Have a feeling that read on here a while back someone mentioning that the new diapraghms could be quite inferior to older ones depending on where you bought them?
Original symptoms sound a bit like teh fuel tank not being able to breathe - thus forming a vacuum requiring greater suction - not changed your fuel filler cap lately?

I don't think I can really say who I bought the new diaphragm from. It was one of the major suppliers. All I can say is that it may be worth watching eBay for NOS ones instead...

It was definitely the pump at fault because it even did it on the bench. It would pump away OK with the inlet and outlet open to the air, then you put your thumb over the outlet and it stalled. Give the body a tap and it started up again. Since swapping back to the old diaphragm it did over 600 miles this weekend without a hiccup.

[bmcecosse]

I've used many an old SU pump - and not had to renew a diaphragm yet. Leave well alone I think is the message here.

[Willie]

It depends who fitted the new diaphragm as it has to be finally clamped
in position only when it is in the 'operated' position i.e. in its stretched
position. I had no problems when I re built my pump.

[alex_holden]

It depends who fitted the new diaphragm as it has to be finally clamped in position only when it is in the 'operated' position i.e. in its stretched position. I had no problems when I re built my pump.

I did that. As I said the pump I rebuilt using a NOS diaphragm is fine and this pump was fine after I put the old diaphragm back. The problem appears to be with the dimensions of the metal disc part of the new diaphragm.