As ahicks suggested, my first suspect would also be the throttle plate. You will have to remove the carb to inspect this and when closed the throttle plate should seal well in the carb bore. Every gas engine I have owned could be killed by closing the throttle completely so closing it enough to achieve a slow idle should not be a problem. If you have to adjust the plate, loosen the retaining screw just enough so the plate can be adjusted ... maybe 1/8 or 1/4 turn. If you go any further, you risk damaging the throttle shaft as the threads on the screw are swaged. After tightening the screw, you can put a tiny dot of green LocTite on the threads if desired.

From there, the only reason for an engine not slowing down enough is an air leak. Make sure all the bolts are tight on the engine and when you inspect the carb, also inspect all the gaskets and the reed assembly. In rare cases, I have heard of a cracked reed housing causing problems such as this.