Foamy gas from fuel pump

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Air Doug
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Foamy gas from fuel pump

Post by Air Doug » Fri Feb 25, 2022 3:02 pm

I finally got the carb off (thanks Chuck W for the bent 1/2" wrench tip!). The previous owner ran it without a fuel filter right before selling it to me. I saw crud in the fuel filter bowl and return flow orifice at the carb. So I know that I have the potential for crud in the fuel system still. I also replaced the fuel pump because I had gas in the crankcase. To check for particles in the fuel lines downstream from the fuel filter, I put a clear vinyl hose on the outlet line (high pressure side) from the fuel pump and cranked the engine into a clear empty bottle. The fuel pump produced a strong, steady stream of gas for the first cup or so. Then the fuel in the tubing got really foamy and the volume of fuel going into the bottle slowed. It seemed to get foamier until I stopped after about two more cups of gas.

So it seems like there is probably an air leak upstream (low-pressure side) from the fuel pump. Fuel pump is a John Emery rebuilt unit with zero hours. Is it possible that there is a leak in the diaphragm? Or somewhere else? How would I check for that? Or what else could it be that I am missing?


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major519
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Re: Foamy gas from fuel pump

Post by major519 » Fri Feb 25, 2022 5:31 pm

I had an issue with fuel on one of my mules. It started taking longer and longer to start due to lack of prime to the carb.
I traced it down to an air leak in the aftermarket fuel
filter the previous owner has installed. Once I fixed that problem solved.
Your description sounds similar with the issue you have. Don’t discount the possibility that you have a bad pump, rebuilt or not. Turning wrenches for years now, I’ve run in issues with brand new or rebuilt parts fresh from the supplier.
1968 M274A5 USMC 325489
1966 M274A2 USMC 333007
1958 M274 US ARMY 2B3379
1970 M38A1 CDN3 70-08715
1981 M101CDN Manac 3/4 trailer
1984 M1009 CUCV
1943 Converto Airborne Trailer

Air Doug
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Re: Foamy gas from fuel pump

Post by Air Doug » Mon Feb 28, 2022 9:11 am

I checked the flow the next morning and it seemed fine then. No idea what was going on before. So I went ahead and installed the new (rebuilt) carb and it is getting gas - the accelerator pump gives nice little squirts when pulling the throttle cable. For now I am considering the foaming problem solved. Now if I can just get the mule to fire!


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