2500 RPM engine runs 'uneven'

Sorry for the 'uneven' term but let me explain. 5 yrs ago I built a stroker for my '76 Stingray. Here's the basic setup:
* Eagle Forged rotating assembly
* 6" H-Beam rods
* SRP Pistons
* Crane hydraulic flat-tappet cam
* Crane 1.6 roller rockers
* Hedman Headers 1 5/8" long tube
* Mallory Billet distributor

For years I have been driving it with cast iron heads as built. They were ported & polished and I've used a Performer manifold on top. Several different carburetors over the years from Q-jets (custom tuned) to different Edlebrocks.

I noticed that with the engine warming up at high idle, it would not stay at a consistent RPM longer than a half minute or so, without very noticeable fluctuation. I noticed this when it had a Q jet on it about two yrs ago. With different carbs, the issue has remained. Now running a Thunder AVS 650 Edlebrock and just like before, at a high idle you will hear it go up & down a bit. Not a misfire like it's dropping a cylinder, just an uneven speed.

Last week I put on AFR 195 heads. Different plugs for these heads, but kept the billet dist and the AVS Thunder carb. Started it up last night and noticed the exact same thing as before, with the old heads: The speed fluctuates when you hold it at high idle, whether it's hot or cold.

Idle is good...800 RPM or whatever I wanna set it at. No problem. I fully expected that with new heads, this problem would go away but it hasn't. The engine produced good power with the iron heads (Dyno tuned @ 275 RWHP) and I am anxious to get it on the dyno with the new heads.

But not knowing what's causing this fluctuation is driving me nuts! Several different carbs, two distributors (did it with the old HEI, too) and now new heads. Suggestions?


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Provided that you're not mistaking the cam "lope" for the uneven fast idle, you might try disconnecting the vacuum advance and see if it evens out.

Same thing w/the mechanical advance- try putting heavy springs on it temporarily so you can see if the advance is the cause of the idle "hunting".

Check all the vacuum lines again- especially the one down to the modulator. Plug it at the mod. and see if it will hold a vacuum from the intake end.

Check the vacuum modulator to be SURE that the diaphragm's holding a vacuum.

It's not the vacuum advance; the problem existed before the Mallory ign when I was using the old HEI. Checking the modulator is certainly a great idea - and Heaven knows there's a ton of vacuum circuits under that hood to control the flip headlights on the Stingray.

Thanks for the suggestions! I can actually cap off all external vacuum ports and run it up to 2500 and see what happens.

Man, I hadn't thought about that set-up in a long time!

But, yeah. I hope you get lucky!