76 W100 fuel problem ?

[04 Diesel]

My truck when cold starts just fine but when hot it takes a lot of cranking, I noticed that when hot the fuel in the fuel filter slowly disappears. I can see bubbles in the filter as the fuel level in the filter drops. Then as the motor cools the fuel comes back in the fuel filter, what is going on? Where is the fuel going?

[Ron in Indiana]

Just a thought but I'd check the float level in the carb and beyond that the fuel pump.

[04 Diesel]

Just a thought but I'd check the float level in the carb and beyond that the fuel pump.

I just rebuilt the carb, the fuel pump is new. Even if the float is set wrong, how would that effect my problem? Not trying to be smart.

[Gordon Maney]

My truck when cold starts just fine but when hot it takes a lot of cranking, I noticed that when hot the fuel in the fuel filter slowly disappears. I can see bubbles in the filter as the fuel level in the filter drops. Then as the motor cools the fuel comes back in the fuel filter, what is going on? Where is the fuel going?

Taking your comments literally, the carburetor is too hot. The fuel is evaporating.

Does this engine have a heat riser valve, and if so, is it stuck shut?

[04 Diesel]

Taking your comments literally, the carburetor is too hot. The fuel is evaporating.

Does this engine have a heat riser valve, and if so, is it stuck shut?

Yes it does, I can turn it by hand, how do I know if its working or not? Are you talking about the metal valve that is on the end of the exhaust manifold?

[dougdodgeman]

Gordon and Ron, I also saw this happen when Karl was here on Saturday. I'm pretty sure after talking with him, that his exhaust heat cross over passage is clogged / blocked. I also pulled the air cleaner lid after it sat at least an hour, and it still had plenty of fuel to give a great accelerator pump shot. I wondered if his tanks aren't venting properly, if there could be a vacuum present when he shuts the engine off, that pulls the fuel back to the tank? Assuming that is what happened, I can't think of a way for the fuel to reappear in the fuel filter a little while latter, without turning the engine over, because there would be no vacuum to pull the fuel back to the filter, or would there be? Doug

[Ron in Indiana]

Best way to see if that's it is to pop the gas cap after driving it. Hearing suction at the cap will verify a non vent situation. I forgot he had rebuilt the carb and a new fuel pump added. 04, drive the truck until warm where it's hard to start. Stop open the gas cap then watch your filter and restart your truck. Might be just a new gas cap to solve your issue.

[Gordon Maney]

Gordon and Ron, I also saw this happen when Karl was here on Saturday. I'm pretty sure after talking with him, that his exhaust heat cross over passage is clogged / blocked. I also pulled the air cleaner lid after it sat at least an hour, and it still had plenty of fuel to give a great accelerator pump shot. I wondered if his tanks aren't venting properly, if there could be a vacuum present when he shuts the engine off, that pulls the fuel back to the tank? Assuming that is what happened, I can't think of a way for the fuel to reappear in the fuel filter a little while latter, without turning the engine over, because there would be no vacuum to pull the fuel back to the filter, or would there be? Doug

Then if the passage is blocked it won't heat the carburetor.

[04 Diesel]

Best way to see if that's it is to pop the gas cap after driving it. Hearing suction at the cap will verify a non vent situation. I forgot he had rebuilt the carb and a new fuel pump added. 04, drive the truck until warm where it's hard to start. Stop open the gas cap then watch your filter and restart your truck. Might be just a new gas cap to solve your issue.

I also did that, I got home from Dougs house and took the gas caps off both tanks and there is no vacuum.

[Gordon Maney]

When hot is the choke wide open?

[dougdodgeman]

It was Saturday with an outside temp in the low 80's

[04 Diesel]

When hot is the choke wide open?

I will check that , but how would that make the fuel in the filter disappear ? Then come back?

[04 Diesel]

Still having this problem, may be a little worse. I now put a 4 barrell carter 600 on my truck, moved my fuel line away from the motor and problem is still there. I was hoping that problem would be better with the cold weather but it is not. Any thoughts?

[76WAGON200]

Sounds like heat soak. A very common issue on carbs with cast iron manifolds and heat crossovers. Some very good suggestions from the boys in here.

Here are a few more:

1) Check the float bowl vent on the carb is not plugged. If you think about it, a pressure build up in the float bowl will do exactly what you are describing. When it is hot, the pressure builds up and forces the fuel back toward the tank. When it cools, the fuel bowl equalizes and fuel flows back again. Some of the ports on certain carbs are not meant to be plugged. Make sure you know which ones are for what. Google is your friend.

2) Install a one way check valve between the pump and carb to prevent the fuel from draining back. Also cheap and a good way to check effect.

3) Install a carb spacer to insulate from the hot manifold. When the intake is hot it boils off the fuel in the carb which breaks the fluid lock in the line, allowing it to drain back. There are super thick base gaskets available that work well. Helps throttle response and is a cheap thing to try.


Good luck!

[04 Diesel]

Sounds like heat soak. A very common issue on carbs with cast iron manifolds and heat crossovers. Some very good suggestions from the boys in here.

Here are a few more:

1) Check the float bowl vent on the carb is not plugged. If you think about it, a pressure build up in the float bowl will do exactly what you are describing. When it is hot, the pressure builds up and forces the fuel back toward the tank. When it cools, the fuel bowl equalizes and fuel flows back again. Some of the ports on certain carbs are not meant to be plugged. Make sure you know which ones are for what. Google is your friend.

2) Install a one way check valve between the pump and carb to prevent the fuel from draining back. Also cheap and a good way to check effect.

3) Install a carb spacer to insulate from the hot manifold. When the intake is hot it boils off the fuel in the carb which breaks the fluid lock in the line, allowing it to drain back. There are super thick base gaskets available that work well. Helps throttle response and is a cheap thing to try.


Good luck!

I have a aluminum offenhauser intake and a different carb, I guess it is heat. I even took my fuel line off the motor and ran it to the side up the inside of the fender. I will try a check valve and a carb spacer, I dont like when I have to crank and crank when its hot. It starts right up when its cold.