Fuel Pump Troubleshooting Question

[sdemore]

All right guys, I'm having a problem and looking for any secret tips for the mechanical fuel pump.

Engine wouldn't start and I got it down to a fuel issue. Started looking and it wasn't moving any fuel from the tank to the carb. The lever on the fuel pump feels "funny." I can't really describe it, but it doesn't feel like it is moving as far as it should be.

I pulled the pump off, disassembled it and all looks good. I just rebuilt it about this time last year and it has less than 10 hours since then. I reassembled the pump and it felt better in my hand. Reinstalled it then tried to refill the bowl with no luck. I put a vacuum pump on the fuel line and it seems to be allowing the fuel to flow, so I am really thinking it is in the pump. I didn't have a fitting to go straight from the pump to a gas can, but will tomorrow.

Everything looks good, seals are in good shape, springs and caps look good, diaphragm is moving freely. There doesn't seem to be any warping above the bowl either. Are there any secret things that I may not know about?

I'll be back there in the morning to do some more work on it.

Thanks,
Steve

[Administrator]

Hi, Steve:

I know next to nothing about these pumps, but is this the kind of behavior you might expect if the pump were starving for fuel from the tank side? Clogged tank vent, clogged tank pick-up tube, shut-off valve issues, etc.?

HTH.

Bill

[Dave Neptune]

Steve, to add to Bill's statement it could be a very minor air leak upstream too.

About the only thing that will keep a mechanical pump from pumping is lack of motion IE the plunger that actuates the pump is missing or worn out or the check valves are in backwards.

If you can actuate the bail and get no pumping it could be either scenario.

Dave Neptune

[joe_db]

Easy to check - hook it up to another tank like an outboard tank.

[zellerj]

could also be a pin hole leak in the pick up tube, or some air leak in the supply side.

Jim

[sdemore]

Thanks for all the suggestions. My first thought was that it was a plugged screen in the fuel tank, but sort of ruled that out. I tried to hook it up to a gas can last night, but didn't have a fitting for the fuel pump inlet. I did connect it at the filter and couldn't get flow there. Went back this morning with a fitting, a piece of tubing, and the gas can. Hooked it up and the pump wouldn't pull fuel from the can.

Disassembled the pump again and everything looked good. Cleaned it up and tried blowing through the inlet, but nothing. Pulled the two little rubber valves and found them both stuck closed. A little PB Blaster and a gentle prod with a small screwdriver and they were functioning perfectly. Reassembled and tested it, and found it to be working. Reinstalled it, hooked everything up, and it is working as good as new.

[capnward]

Glad you found the problem. As another mechanical fuel pump owner, I noticed your statement that the two little rubber valves were stuck. My question is why did that happen? In 2500 hours, that has never happened to me, and I use MMO in non-ethanol fuel. My opinion (I like to call it a theory, but others might call it a superstition) is that ethanol degrades the rubber in the valves and diaphragm of a mechanical pump, which makes the valves sticky. MMO keeps them lubricated. If you are not using MMO in your fuel, I recommend that you do, especially if you are using 10% ethanol.

[sdemore]

I do use MMO in the fuel (and oil), but haven't been able to find non-ethanol fuel locally yet. I've been working on the boat and wound up going for about 2 months without starting it. Not sure if that was the whole problem or not.