M151 gas evaporating from carb

jeffhuey1n

SMSgt, USAF (Ret.)
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I’ve come across this problem. It drove me nuts until I did some serious digging into some really old motor manuals. First off, trace the fuel line from the tank up to the carb. Make sure there are no kinks, bends, pinched lines or any other mechanical problems. Next, since you have a carb problem, pay very close attention to the fuel line once it enters the engine bay (compartment). Is the fuel line touching anything hot! The exhaust manifold is a serious problem to fuel vaporization. Make sure there is a gap between the fuel line to any metal engine parts. My specific problem arose from the metal bodied fuel filter coming in contact with the engine. Even rerouting didn’t work for me because I didn’t have a method for putting a cooling feature on the line. By cooling feature, I mean putting a shield between the line and the hot engine. If you have some back yard engineering skills, making one up is quite easy. I used a piece of thin steel sheet and attached it to the fuel line so that heat from the engine couldn’t reach the line.
I hope this helps.
 

M1078MAN

Active member
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SW Va, Roanoke County
I’ve come across this problem. It drove me nuts until I did some serious digging into some really old motor manuals. First off, trace the fuel line from the tank up to the carb. Make sure there are no kinks, bends, pinched lines or any other mechanical problems. Next, since you have a carb problem, pay very close attention to the fuel line once it enters the engine bay (compartment). Is the fuel line touching anything hot! The exhaust manifold is a serious problem to fuel vaporization. Make sure there is a gap between the fuel line to any metal engine parts. My specific problem arose from the metal bodied fuel filter coming in contact with the engine. Even rerouting didn’t work for me because I didn’t have a method for putting a cooling feature on the line. By cooling feature, I mean putting a shield between the line and the hot engine. If you have some back yard engineering skills, making one up is quite easy. I used a piece of thin steel sheet and attached it to the fuel line so that heat from the engine couldn’t reach the line.
I hope this helps.
Spot on, called vapor lock
 

tennmogger

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Greenback, TN
Or it could be gas evaporating out of the carb :) Many carbs have a fiber or other anti-heat-wicking barrier between the carb and the intake manifold. Does the '151 have one? Maybe add one?

On my 404 Unimogs this is a common problem. The gasoline now-adays is very volatile and evaporates easily. If you run the vehicle then shut it down, or just stop for a while, the gas boils/evaporates out of the carb and starting is slow. Adding an electric priming pump (at the tank) to fill the carb is a good solution. That is a cure for vapor locking too. I have also added some Diesel to the gas to drop the boiling point (10% Diesel) but that's a decision you have to make. The older engines run just fine on lower octane gas.
 

nattieleather

Active member
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Cleveland, OH
On the m151 the fuel line come in straight to the carb and is on the same plane as the exhaust manifold. The Army did an MWO wher they put a 45 degree elbow at the carb inlet so that the fuel filter which was right there would be kicked up and out of the plane of the manifold thus in theory stopping the vapor lock. If you don't have one of those 45s try getting one and installing it and see how that goes.
 
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