I checked out a 1980 Corvette today, and the throttle was doing the same thing. Someone had backed the throttle stop screw all the way off, and it felt like the throttle plate was sticking in the bores. The mixture screws were adjusted almost all the way out to compensate for a decent idle.
At first I thought of weak throttle return springs, but after adjustments it came right back down to the proper idle speed.
This might be something else to check.