Idle drops occasionally when warm
 

Hello,

I have a '65 tri-power set up on a 455 M/T.

The idle has always been smooth at about 850rmp running both pump gas and race gas at all temperatures.

Recently, when the engine is warm, the idle will occasionally drop down to 550-600rpm at stoplights and sounds rough. This only happens when the engine is warm. I switched to non-ethanol 100 octane today and it still happened at a stop light after 25 minutes of freeway driving. I put on the parking brake and revved the engine and it returned to an 850rpm idle. It happens on an incline more than flat, but today was flat. It doesn't always return to an 850rpm idle.

This has been happening for a couple of months. Other than that, the engine runs great.

Any ideas?

Float settings, fuel pressure, non-vented bowl to throttleplate gaskets, non- functional hot idle compensator comes to mind.

For fuel pressure, I think I would have a host of problems whether too much or too little fuel pressure.

Also, I would expect the problems to be all the time, not intermittent.

For the float level, the same thing.

The hot air idle compensator, I didn't think of that. I will look at it. What should I look for? Is it adjustable?

Here's a photo.

That photo is of an idle vent valve. The hot idle compensator (if you have one) will be on the rear of the center carb above the vacuum hole. It is a raised section with a screwed on plate and a small spring loaded button in the center. They are not adjustable but the little button is to be sure the valve is closed while adjusting the carb idle. They probably only came on cars with AT and AC.