The basic engine is a simple air pump, That thing is getting un adjusted air somewhere causing the idle to stay high. Also causing a lean condition most likely. All the carburetor is is an air fuel metering device. and the intake dispurses that air fuel mix evenly through out the engine, any air entering the engine in any other way is a LEAK, and causing a high idle, unless you have the timming all jacked up.

More than likely an air leak.

Which has already been stated.

I have had a stuck pcv valve do this,and have had a brake booster check valve/gromet do this.

I have also had the small 318 port size tin gaskets leake with a 360 port size intake,from under the intake,burning oil/smoke showed up right away.

had a carb not torqued down give fits for a couple days,went to R&R it and found the nuts loose

I wager vac leak but I have also had to reset carb settings/adjustments when swaping from engine to engine.

get a vac gauge and timeing light on it and check for leaks