AC Delco Fuel Pump Assembly

[david.g.chapman]

I read recently that when assembling the diaphragm on these pumps, you should bias the rocker arm upwards to bring the diaphragm down level with the pump flanges while you tighten the flange bolts.

Needless to say, I did not do that when I refurbished and the pump is back on the car, and working OK.

After some head scratching as to why you need to do this I reasoned that the diaphragm must be stretched upwards on assembly if you leave the rocker arm down, This puts slack in the diaphragm when the pump is working which might affect the pumping action.

I am telling myself that the replacement red diaphragm is elastic rubber rather that the old composite black diaphragm, so this does not matter so much. I can see the edge of the new diaphragm is flush with the outside of the flanges. So should I leave well alone or re-assemble the pump (which means taking it out again) ?

Dave Chapman

[William2]

The pump arm should be in a position whereby the diaphragm is exactly level with the body flange to ensure the pump works at maximum efficiency. I rebuilt mine recently as I had trouble getting it to pump correctly.

[dougal9887]

I wasn't aware of this when I rebuilt the pump recently. I have a slight misfire at near maximum revs in 3rd and more so in 4th which I suspect is due to fuel starvation. I was going to change to an electric pump but will now correct the assembly of the diaphragm.
Or maybe not since I would also like the carbs to prime before starting. Or maybe the carbs would fill more quickly if the diaphragm installation was corrected.
Dougal.

[david.g.chapman]

I had trouble starting the engine yesterday before I read about my diaphragm assembly error. I checked the fuel pump by disconnecting the pipe to my stromberg's T- piece and turning the engine over. There was a good burst of fuel ejected at each stroke of the pump, so I think I will leave it alone. (the engine started eventually - old fuel I think)

After all if you can get the six mounting screws through the diaphragm holes it can't be far out. I believe the resting point of the diaphragm should be at the level of the flanges when the rocker arm is halfway up the jackshaft cam. There is not much that can be adjusted though...

If there is a next time I will try and remember!

Late edit - I looked at my old black AC diaphragm, and it was domed in the center with significant slack. As there is no stretch in the fabric I think it was made like that. The action must be like a bellows with the diaphragm flicking from concave to convex in a cycle. With the new red rubber diaphragm there is no slack - have a look on the small ford spares website:

https://www.smallfordspares.co.uk/produ ... el-e36-z-1

- you get a flat disc. So in this case the diaphragm stretches a bit during each cycle. Interesting... I really must get out more..

Cheers,

Dave Chapman.