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BF90 needs fuel pumps?

J

jrude

Member
It runs awesome at high RPM, IE 3500 RPM and above. At idle, it runs fantastic, until it runs out of fuel, then it starts to stumble.

If I start pumping the fuel bulb right when it begins stumbling at idle, the stumble goes away and it idles fantastic. Fuel bulb is soft after idling for a while. Getting the engine back up to speed, no more soft fuel bulb, and no more stumbling.

Ran the engine WOT for at least 1 hour and 20 minutes with no problems. Burned almost 5 gallons of fuel. Run it at idle for 3 minutes, engine stumbles and will stall.

https://www.youtube.com/watch?v=G5ESbKiFi_E


Fuel pumps are about $150 so I'm going to gamble on them, but just want to check if it could be something else I'm missing. I don't think there are any vacuum leaks in the lines.

Thanks
 
hOW CAN IT BE BE THE FUEL PUMP WHEN YOU RUN FULL THROTTLE FOR A LONG PERIOD OF TIME ,START WITH A NEW BULB, YOU REALLY NEED A COMPUTER DIAGNOSTIC TO PINPOINT THE PROBLEM!
 
The engine has carburetors. No computer. The fuel pump and fuel line are brand new, but that does not rule them out as a problem. Thanks.

My belief is that the pumps will not pull enough fuel volume at low speed, but at high speed they're working much faster, and pulling enough to keep the engine running.
 
Fuel pumps arrived yesterday and I installed them this evening. I let the engine idle in the water tub for about 20 minutes and the engine never stumbled at all.

When I held my finger over the ports and depressed the plunger on the new pumps I could feel vacuum or pressure on every plunger stroke. On the old pumps I can only feel vacuum on the inlet side I never felt pressure on the outlet side. I am 99% certain the fuel pumps will fix the problem, but I will not know for sure until Saturday.

If you have done this job before you will know that the bottom bolt on the bottom fuel pump is a real pain in the butt. If you have not done this before it is pretty easy to get that bottom bolt out if you use a socket and a drill adapter and a quarter inch wrench.
 
The fuel pumps fixed the problem. I pumped the fuel bulb during the first start cycle and that was the only time. The engine idled for about 2 hours, and ran three-quarter throttle for about an hour. The stumbling problem is gone. I probably started the engine 15 times total.
 
Last edited:
Thanks. I was thinking about it after the fact, and I believe this is what is happening with the old fuel pumps:

1) At high speed, even with a leaky seal on the pressure side of the pump (that fills the float bowl), the diaphragm in the pump moves fuel faster than the engine can consume it, and that keeps the bowls full.

2) At low speed, the engine consumes fuel very slowly. Since the pressure side of the diaphragm is leaking, when the inlet side of the diaphragm tries to pull fuel from the fuel tank, it actually pulls from both the tank and from the pressure side of the pump (the float bowl). Since the engine is drawing fuel from the float bowl, the fuel level in the bowl deceases, and the float valve eventually opens. Because the pressure side of the pump is leaking, this allows air to pull through the float valve and into the pressure side of the pump, which then pulls through to the inlet side of the pump, which is the fuel tank/fuel bulb. This makes the bulb soft.

Anyway, hope this helps someone else down the road.
 
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