fuel issue

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joe1
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fuel issue

Postby joe1 » Sun Aug 12, 2018 8:55 am

have a 1948 cub with a strange problem
when tractor is running the fuel is being blown back into the fuel tank; causing the tractor to run out of gas; even when I turn off I can see and hear bubbles in the tank. could this be a intake valve leaking?

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Re: fuel issue

Postby Cubfriend » Sun Aug 12, 2018 9:04 am

Sounds like compression is coming back through the manifold,carburetor and fuel line to the tank. Check compression to narrow down which valve it is and see if a valve adjustment can alleviate the problem.
Frank

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Re: fuel issue

Postby Crimson Tim » Sun Aug 12, 2018 9:32 am

I don’t think it can do that, can it? Even if you had leakage past an intake valve, wouldn’t the intake draw of the other cylinders be greater that any back flow?
In any case a valve adjustment would be a good thing to check, and a compression check will tell you for sure if you have a valve issue.

It sounds to me like there is a blockage in the fuel line, though. At least that is the first thing I would check. It traps a little air when you turn the fuel on, which then works it’s way loose and bubbles back into the tank. And then because you have a fuel restriction, you run out of gas after running briefly.
Do you have any rust in the tank? It doesn’t take much to cause problems.
Check the whole fuel line from tank to carb. Especially the sediment bowl screen, the screen at the inlet to the carb, the main jet in the carb. It may even be necessary to take the sediment bowl off and flush it out. There’s a jog in the fuel path at the valve and before the screen that can collect stuff.

Also try running with the gas cap ajar. If the vent in the cap is plugged, you’ll end up with a vacuum in the tank, which would slow/stop fuel flow and might be able to suck some air back through the line before the pressure equalizes?

Very curious. I’ll be eager to see what the problem turns out to be!

joe1
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Re: fuel issue

Postby joe1 » Sun Aug 12, 2018 10:09 am

I am going to check comp; I already checked the fuel path from the tank to the carb; 100% clean; I have an in line gas filter that is clear and when it does it I can see the gas being forced back into the tank with the fill cap off and bubbles coming out the tank exit. and not a little its like your blowing air through the line. I didn't think it was possible either

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Re: fuel issue

Postby tst » Sun Aug 12, 2018 11:02 am

a bad intake valve would make it spit back through the carb, does the engine run on all 4 cylinders? I have seen a couple where the valve keepers fell off and the valve will not open causing it to run very erratic as it can not release the pressure through the valves

joe1
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Re: fuel issue

Postby joe1 » Sun Aug 12, 2018 11:35 am

Checked the comp dry each cyl had 95- 100 ;then I pressurize each cyl one at a time no bubbles the problem is intermittent; when its not blowing bubbles it starts and runs excellent; no skipping and plenty of power

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Re: fuel issue

Postby Jim Becker » Sun Aug 12, 2018 12:37 pm

Does the gas line run down hill all the way from the tank to the carburetor? An S bend in the line can trap an air bubble that can cause odd (and intermittent) problems. Does any part of the line touch (or run very) near the exhaust system? Your problem could be from boiling gas in the line.

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Re: fuel issue

Postby Crimson Tim » Mon Aug 13, 2018 10:31 pm

If it’s intermittent... a sticky intake valve?

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Re: fuel issue

Postby inairam » Tue Aug 14, 2018 5:41 am

Is it possible that the fuel line is running against the exhaust which is heating the fuel causing it to boil backward to the bowl and tank?
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Re: fuel issue

Postby radioguy41 » Tue Aug 14, 2018 6:47 am

That was my thought too, the fuel boiling from the line being too close to the exhaust. In the old days it was called vapor lock.
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