C2:67 Heat Riser

[Gerard F.]

There's probably a simple answer to this, but how do you tell whether your heat riser is working properly? I'm getting some erratic behavior with this restored carb and I have plugged the heat slot. Sometimes it runs hot at slow speeds, sometimes it dies when hot and sometimes it idles high. At high speed it gives you a kick in the butt and runs perfectly.

Jerry Fuccillo
#42179

[Stephen L.]

Re: C2:67 Heat Riser

Determine the position of the weight on the outside of the heatriser when cold and when hot. (Do this when the engine is cold so you don't burn yourself).
Now start it up and watch the heatriser weight to see if it opens. When you are experiencing the "problem" take a look at the weight to see if the heatriser is fully open.

It sounds like your mixture is varying... could you have a manifold air leak that is sensitive to heat... Once you get on it the "leak" to "carb air" ratio becomes insignificant and it runs great. Are the butterfly shafts leaking air at idle?

[Duke W.]

Re: C2:67 Heat Riser

Check (engine cold) that the heat riser valve moves freely by using the "counterweight" as a lever. I think the weight should point to the rear when cold and down when hot, but I might have this reversed. Also verify that it moves 90 degrees when hot relative to cold.

Since most of us only drive our Corvettes in mild weather, I think it's best to wire it open. Some have replaced it with the FI spacer, and some have removed the valve leaving the spring and shaft in place so it looks OE to observation (judging).

Duke

[Gerard F.]

Re: C2:67 Heat Riser

Thanks Duke,

I think I found the problem. The counterweight goes down to open but it doesn't go all the way to full open on mine when it gets hot. When hot and when I give it a blast to opens under exhaust pressure but not all the way. At hot idle it almost goes back to a closed position.

I'm going to try wiring it up and see the difference and then might try your trick with the new exhaust system I've got coming.

Thanks again,

Jerry Fuccillo
#42179