I have a 1969 Super Bee with a 383 335hp motor and 4 speed. It had an older Edelbrock 650cfm carb on it when I bought it. The problem was that the idle would start out at 900rpm (just where I set it at) and after driving it it would idle up to 1400 rpm or so or possibly idle down to 500 or so. Just all on its own. Based on where it might end up idling at, I would adjust the idle screw but that would make it idle even higher or lower, Not a vacume leak. Had the carb rebuilt. Nothing seems to change this. So I bought a new ASV Edlelbrock 650 and it does the same thing. Still running the stock distributer and coil. Any ideas?

 

Have you tried just running the car without any vacuum advance?
Also Make sure the choke is not the issue? as in closing and putting the engine up on the fast idle cam....