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Testing a mechanical fuel pump
https://ausmini.com/forums/viewtopic.php?f=2&t=101616		 Page 1 of 1
Author:  68+86auto [ Sat Dec 03, 2022 9:13 pm ]
Post subject:  Testing a mechanical fuel pump
Is there a good way to test if a mechanical fuel pump (goss) is providing enough flow and pressure?

I am trying to sort out an issue with a mini that I am now thinking it is somehow not getting enough fuel above about 2500rpm. Making the mixture richer works for a couple of quick revs but then backfires.

Is 6 degrees at about 650rpm and about 12 degrees at around 1000rpm likely to give ok timing with a 29d?
It could still be an ignition issue but it doesn't look like it should be this bad on my engine analyser but I am not experienced using it.

Engine details:
1968 998 auto just "rebuilt" by someone doing their first engine build (not me).
Carb: HS4
Distributor: worn out 29D with new points and condenser
Fuel pump: original? Goss pump rebuilt in 2019. When this engine was assembled the pump decided to start leaking but tightening the diaphragm screws fixed it.
Author:  Steam [ Sun Dec 04, 2022 11:37 am ]
Post subject:  Re: Testing a mechanical fuel pump
The pressure should be 2 to 3 psi as stated in the manual. There are pressure gauges available that measure fuel pump pressure. The one I have here is combined with a vacuum gauge tester.
The flow is listed in the manual as approx 30 litres per hour, if you do the math and work out how long it should take to pump 1 litre then you could test it by timing flow into a litre container.
However I would suggest mixture and or timing or distributor issues are your problems.
Author:  Bill B [ Mon Dec 05, 2022 7:07 pm ]
Post subject:  Re: Testing a mechanical fuel pump
Steam wrote:
.......The flow is listed in the manual as approx 30 litres per hour, if you do the math and work out how long it should take to pump 1 litre then you could test it by timing flow into a litre container.

1 litre every 2 minutes?

Once had a similar issue: car idled well, accelerated OK briefly then spluttered and lost power.
Turned out it was a partial blocked needle/seat in the fuel bowl: filled the fuel bowl at idle but flow rate couldn't keep up with higher revs for long.
Author:  68+86auto [ Tue Dec 06, 2022 3:20 pm ]
Post subject:  Re: Testing a mechanical fuel pump
Steam wrote:
The pressure should be 2 to 3 psi as stated in the manual. There are pressure gauges available that measure fuel pump pressure. The one I have here is combined with a vacuum gauge tester.
The flow is listed in the manual as approx 30 litres per hour, if you do the math and work out how long it should take to pump 1 litre then you could test it by timing flow into a litre container.
However I would suggest mixture and or timing or distributor issues are your problems.

It seems it might not be the fuel pump but I haven't tested it, I guess I would have to test it with a small volume and crank it?

So far nothing had fixed the problem until I said to try increase the plug gap from 25 thou to 30 thou as the firing voltage was a bit low ranging from 3kv to 7kv. Now it is running a lot better but still not done yet.

I am "Not allowed" to test the timing at 4000rpm so who knows what the advance is doing.
Author:  Steam [ Tue Dec 06, 2022 5:50 pm ]
Post subject:  Re: Testing a mechanical fuel pump
As a faily quick test for the pump, if you get the engine to the point where the fault occurs then immediately kill the engine you can have a look in the fuel float bowl to determine the level. It should be about half to 2 thirds full with the top off. If it is the pump the bowl will be just about empty.
You may have answered your own question with the worn out 29d, they were not very good distributors when in good nick and it will be scattering the advance more the faster it rotates.
Author:  68+86auto [ Wed Dec 07, 2022 8:47 am ]
Post subject:  Re: Testing a mechanical fuel pump
Steam wrote:
As a faily quick test for the pump, if you get the engine to the point where the fault occurs then immediately kill the engine you can have a look in the fuel float bowl to determine the level. It should be about half to 2 thirds full with the top off. If it is the pump the bowl will be just about empty.
You may have answered your own question with the worn out 29d, they were not very good distributors when in good nick and it will be scattering the advance more the faster it rotates.


It now runs not badly just it is getting hot. I have now tested it at 2500rpm and there was a lot of timing scatter.
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