Track down a vacuum leak?

[VintageMusclecar]

With the engine warm and idling, go around the intake/cylinder head mating area as well as the base of the carb and the throttle shaft areas with a can of carb cleaner. If/when you find the leak the engine will draw the carb cleaner in and you'll hear the engine "clean up" momentarily.

You might also verify that the correct carb mounting gaskets are in place, and that the throttle shaft bore isn't worn loose. Lastly, go over all the vacuum lines to make sure none have fallen off, split, or otherwise deteriorated.

Eric