The Stovebolt Site Forums The Shop Area Engine Shop Exhaust Manifold Heat Riser Valve

What happens when it sticks? I would think this would happen a lot with trucks that have sat for a while.

I had one stick on a '54 IH 1/2 ton with the Silver Diamond 220 engine, and lost one cylinder at a time until it wouldn't pull anymore. When I opened the hood, the carburetor was a solid block of frost.

I assume it was stuck closed. If open is normal, I suspect that sticking open doesn't do anything after the engine is warmed up. But I'm curious. Does sticking in between open and closed have much of an effect?

The function of the heat riser valve is to speed up the warm-up process of the air-fuel mixture for proper atomization. When stuck open it will slow this process, and can also contribute to carb icing if not performing it's designed function during operation. Stuck closed, it can generate too much heat, and you will over temp the carb which will contribute to vapor lock. Generally it's an all or nothing proposition. It will not usually stick half way unless there is some sort of mechanical interference, say from an oil filter line.

My 38 stuck between, mostly in the "heat on" position but not completely closed. I had a perennial hard-to-start-hot issue because there was too much heat in the carb. When I shut it off hot, it would boil the gas out of the bowl and into the intake. It was like trying to start a hot engine with the choke on. After a lot of effort I was able to loosen the heat riser up enough to move it but could never get it free enough for the thermostat spring to work as it should. I ended up jamming it into the heat off position and using the choke to assist with cold running. When the weather is cool I let it sit & idle for a couple of minutes before driving. With the manifolds connected there is plenty of heat in the carb by then.

I just finished taking my whole heat riser assembly off, and tapping the hole and putting plugs in. Not by choice!!. Mine was stuck, and tapping with a hammer, I busted the weight side of the assembly. Don't know if it will make that much of a difference. I will be like Tiny, and just let it warm up a little longer I guess.

Well said Tiny.

By the way, that old car is looking good.

Thanks man. Keep it up, flattery will get you everywhere.

Now I'm curious. My father had a 1962 Jeep Wagoneer, with the flathead 6. We had starting problems until I put an electric fuel pump in it. Always seemed that the float bowl was empty. Even the electric pump would have to run for a few seconds before it lugged down, like it was pushing against a closed needle valve.

Do you think it's possible that the heat riser valve was always closed and the fuel percolated out of the float bowl? We never saw any signs of fuel leaking from the carburetor.