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Advice on diagnosing defective fuel pump....

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guzzul

guzzul

Jedi Warrior
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I am installing a new fuel pump from TRF in my '78 Spitfire (the old one was leaking).

Seems pretty straightforward. But I am not getting fuel pumped to the carb. I have removed and reinstalled a couple of times, ensuring I am getting the pump arm properly riding on the camshaft. I'm pretty sure it is, since I have to 'compress' the arm against the cam lobe in order to get the pump to mount properly on its studs. I can move it in and out against the cam, and seem to get pumping action.

But nothing pumps.

Yes, there's lots of fuel in the tank. And I can get fuel from the tank to flow with gravity, so no blockage as far as I can see.

I have had the pump out on the bench, and manually pumped the arm with a vacuum guage plugged onto the 'suction' side of the pump. Doesn't seem to create much vacuum. The 'blow' side of the pump seems ok - I can use it to puff small pieces of paper towel around, for example.

I'm not sure what I'm looking for. I guess its possible to have a new pump that is defective. Is there a technique for testing these things? Other than sticking it on the car and seeing if it works?

Any advice or experience would be appreciated.
 
No real way that I know of for test other than install and see if it works. I guess you could try installing a fuel line to it and gravity feed to the inlet of the pump and pump the arm by hand. Check the screws to make sure the top is tight to the bottom of the pump and there are no are air leaks between the pump body flanges where the diaphram seperates the two halves of the pump body. Do not overtighten because they can strip easily and or snap off. I guess you would have seen leakage from this area had fuel actually entered the pump if it were leaking air. If your pump has a sediment bowl is the bowl filling with gas past the pump? It could be the diaphram is stuck or torn internally but I would send it back and ask for a new one before taking it apart. Oh it takes a while for these things to get the fuel to the carb when dry if you're visually waiting for it at the carb inlet. put a short line off the outlet of the pump and see if that shows fuel quicker be careful of sparks and catch the fuel in a container.
 
Try priming the pump. As stated by Harry it takes a while for the fuel to get to the carbs. Try poring gas into both inlet and outlets. If you have a pressure gauge hook it up check the pressure side. Should be around 5 lb I'd guess. If still nothing send it back!
 
I can't take the pump apart - it's a new unit and it's all one sealed piece. So there are no air leak areas. My old pump was like the one you're describing, and indeed had a leaking diaphragm. If I could find a new diaphragm I could just repair the old pump.

I previously tried pulling the outlet hose at the carburetor, and putting it in a jar to see if the pump is putting anything out. Completely dry.

I will try priming it as suggested, and see if that makes a difference.

Thanks.
 
My replacement pump is a two piece set up almost as original except without the hand primer. Any chance they sent you the wrong pump?
 
On some pumps( AC BRAND in particular )they required a 1/2"spacer gasket on the later Spitfires,Maybe you should look at the discription & diagram in the Victoria British catalog for the proper fuel pump for the 78.The reach length of the arm is different lengths on EARLY & LATER PUMPS.The EARLY pumps had a primer lever /ubbthreads/images/graemlins/hammer.gif & the arm was a 2-piece arm,The AC PUMP had a SOLID arm,THIS could be your problem THE WRONG PUMP. /ubbthreads/images/graemlins/shocked.gif
 
I don't think its the wrong one, but it is not the same as the originals. It fits perfectly, and functionally it should do the job, its just not a 2-piece unit. TRF sells these as a plug-replacement.

I just phoned TRF today, and the tech support guy says it should work, but if it doesn't there is not much to do but send it back. He stressed the need for good gravity-feed to get it going, and he indicated the 'suction' side does not create much real vacuum. It relies a lot on the tank being above the pump (although I'm not sure what that says about driving up-hill!). He also confirmed there is not much chance of mounting the pump wrong (i.e. under the cam versus against) on ths model Spit.

Given that input, I am going to do as you and TR3ATR250 suggest, namely 'prime' it with gas, and try again.

If that doesn't work, it's going back.
 
If it is the correct part then there is precious few things that can go wrong... simply because there aren't very many parts in that guy.

One thing that can fail (and thus could be defective from new) are the check valves. One only lets fuel in, the other only lets it out. If these are operating correctly you should be able to blow (but not suck) on the inlet... and suck (but not blow) on the outlet.

Try to get as much gas out of it before doing this... even then you will be wanting to brush your teeth real soon afterwards.
 
Thanks. As per my post to Harry Ward, I think its the right one, albeit a plug-replacement, and not the orginal design.

This one does not have a spacer. Some Spitfire models used a 'long-arm' pump, and a spacer, but this one is the short-arm version, and mounts directly against the crankcase. The pumping lobe on the camshaft is right there just inside the mounting hole, and I can't see how that could be wrong.

I am going to try priming it, the feed line from the tank may have drained a bit when I pulled the old one. We'll see what happens.
 
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