[jhv48]

Before you get too deep into this, check one very simple thing:

With the engine fully warm and parked, disconnect the linkage and throttle cable and manually rev the engine (at the carb) to about 2500 RPM and then VERY SLOWLY let the carb linkage return to idle.

I'm betting the carb will not return to idle but will hang up somewhere.

If it does, BY HAND, grasp the throttle linkage at the carb and see if you can manually close the throttle. AT THE CARB!

If you can close the throttle so the car returns to normal idle, you have narrowed down the problem to the throttle shaft or maybe some deposits are preventing the butterflies from completely closing, thus binding slightly.

If this happens, take the carb off and turn it uside down. Now look through the bore and shine a flashlight in the bore from the opposite side. Manually open and close the butterflies and look for even light around the entire throttle plate. If a dark spot appears, it means it is hitting the bore and could be slightly binding at that point. Sometimes a simple adjustment is all that is needed. Possibly needs cleaning too. Either are very easy to do.

Good luck.