It looks like this was a case of the throttle linkage not returning all the way. I put a stiffer spring on and it solved the problem nicely. I did pull the carb and check the butterfly valve, the bushings, and the lever. The valve isn't sticking in the throat, no play in the lever, and there is a very small amount of wear in the bushings. I noticed that when the engine would stick on its usual slightly high idle, I could reach down, grab the accelerator pedal with my hand, and pull it up gently and that corrected the idle. So I just put in a stiffer spring. That seems to have done the trick. No binding at the firewall, by the way. Maybe the throttle return spring was just too weak.
