fuel drainback?
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Senior User
Joined: Apr 2005
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From: South Bend, IN
fuel drainback?
My unmodified '76 F-250 with a 300-6 and in-cab fuel tank has a problem with fuel drainback or evaporation after sitting several days. It starts, idles, and runs great, but if you let it sit for several days you have to crank for a really long time to get fuel back into it so it will start. (4 or 5 10 second bursts). Then when it gets gas into it again it takes right off suddenly and runs great. The carb was rebuilt by the PO, fuel pump is original, rubber and steel lines have all been replaced by PO (and they look newer than the rest of the truck). Does the Carter 1bbl carb have that dreaded 'power valve' that leaks down that I've heard about? It runs so good when it runs that it's hard to believe there's anything seriously wrong anywhere. The crankcase oil level is not increasing so I don't think that it's leaking gas down into the engine. I'm about ready to put an outboard-motor style primer bulb in the line that I have laying around, and just give it a squeeze or two before I try to start it, but you folks that read this forum know that I HATE hacking things up like this. Ideas? Thanks, Brian
Senior User
Joined: Jun 2003
Posts: 286
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From: Ousley, GA
My '74 with a 4bbl Holley, my '76 with a MC 2100 and my '84 CJ7 with a 2bbl Weber (soon to be MC2100) all have this problem. The chokes all work well and don't require a LOT of extra cranking but after sitting for a while the cranking time is extended to get the fuel back to the carb (from 1-2 seconds to 5-8 seconds). I'm guessing because it's not a closed loop system (like fuel injection). I'm sure someone here will have a better explanation and perhaps a solution.
Postmaster
Joined: Apr 2000
Posts: 2,838
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From: SD
It should not do that. If the fuel system is sealed, it should not leak back. You have a small air leak near the engine, causing the fuel to drain back to the tank. Most common problems are dry cracked rubber hose or bad fuel pump. Since the rest has been replaced, I'd suspect the fuel pump. They are cheap and easy to replace, I'd start with that.
Marty
Marty
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Joined: Nov 2004
Posts: 445
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From: Vancouver, Canada
Pardon my ignorance about all these different carbs/fuel systems in different trucks mentioned so far, but doesn't each carb have a float bowl? Shouldn't a little bit of fuel remain in the bowl for a month or so? My 76 F250 with 460 starts quickly if parked up to about 5 days but has to be cranked for 10 - 20 seconds if parked longer - so I have the same problem.
Posting Guru
Joined: Jul 2003
Posts: 1,280
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From: alaska
you possibly have a float bowl gasket leak, which can sometimes be fixed as simply as tightening the bowl screws. which worked on mine because the fuel was evaporating , another fix is to switch to an electric fuel pump, my personal favorite on my 76 one ton, I tightened the screws , which fixed it but I wanted to put an end to long cranking , which eats starters, my 2c's, bob
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