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