In doing some of what Hotrod suggested, I removed the carb from the manifold. While I had it disconnected from the linkage, I depressed the pedal to see how it responded. To my surprise, it did not return completely (I had to pull on the top of the pedal to get it back all the way), so it apparently has nothing to do with the carb itself. I had previously noticed that where the shaft goes through the firewall, it was a pretty tight fit, and on closer inspection it appears
this is where the problem is. I don't see any way to adjust the location of the metal flange surrounding the shaft, as it's riveted in place.
