The Stovebolt Site Forums The Shop Area Engine Shop 235 -- 2 mile skip

My 51 truck has a 1961 235 in it. It has been rebuilt including the carb. On the road it will have a slight miss about every 2 miles or so. It doesn't seem to bother the way it runs but it is annoying. I can pull the choke out about 1/2" and it stops. Seems to me a carb. adjustment would take care of this,would it? This 1 barrel Rochester only has one adjustment screw on the base next to vacuum line. I don't want to try this if it will not help since it runs really good other than this. Any ideas? thanks mh

Is your fuel filter clean ??

Is your heat riser valve stuck?

My rebuilt 235 with the original carb does somthing similar. It may be water in your gas thank. This could explain the randomness of the miss. If you still have the solid lifters they may need some adjusting.

By the way....... my 235 hesitates when I give it gas. The carb was just rebuilt. Does yours do the same?

Hey 4onTheFloor: Where is the heat riser valve and what does it do?

Fuel filter is new so should be good but you never know. The heat riser valve is free no binding at all. Mine did hesitate when you accelerate but it was the vacuum advance now it has no hesitation. The miss is not very pronounced just enough to notice but pulling the choke a little will stop it. I would rather be able to run without choking it. :confused:

It sounds like you may have a small vacuum leak at the manifold to head.(since you need to run with the choke out a little.)

Adjust your carb mix screw so it has about a 25rpm or so drop from peak rpm (called a lean roll)


You can (on older vehicles without sensors)use a little auto trans fluid and dribble it down the carb throat .It will lube the valves and free them up a little to keep them from sticking.

One little problem . . . so many answers . . .

A little side question. I didn't even know my truck had the heat riser valve until you guys mentioned it and I started looking for it. I found it and it moves freely. But the spring seems very weak and I am not sure how this is supposed to work. Does this work by itself? Do you manually close the valve during cold starting and how does it stay closed? How does it then open again? Stupid questions I know...

No stupid questions, just a lot of stupid answers.

The heat riser valve is operated by the spring which is temperature sensitive. When it's cold, it contracts and closes the valve, directing heat to the intake manifold. When the spring heats up, it expands and opens the valve, directing the exhaust out the pipe.

I saw a replacement spring in the Chevy Duty catalog. Is it a good idea to go ahead and replace it? Or can the valve's operation be tested?

Tab- the heat riser valve is in the stock exhaust manifold right below the carb. It it a valve that either sends exhaust out the tailpipe or diverts some up through the intake manifold, around the base of the carb, and back out the exhaust manifold. It helps the mixture be correct when the engine is cold. If it is stuck you experience mixture problems. There is a weight on one side that hangs on the shaft and a coiled spring on the other side.

Tony M-
To check if it is working correctly, start the engine cold and watch the weight. The weight should rotate about 90 degrees as the engine gets hot.