I have a buddy with a 1986 Mercury 150 who is having an issue with the starter and flywheel jamming when trying to crank it. He had a lower unit seize, replaced it with a used one and all was fine until a couple days ago when he tried to run it with the muffs in the driveway. With the lower unit removed, it cranks over fine. The shaft in the lower spins when not installed. Its only when the lower unit is installed that it gets jammed up when trying to start it. Any ideas? Thanks in advance

One would have to assume, it's either the wrong LU and jamming into the crank or the LU is bad.

 

The only other thing I know of is a bad lower main bearing on the crank.  That's a roller bearing and controls the end play on the crank.  That can be checked by prying up and pushing down on the flywheel.  There should be absolutely no up and down movement of the crank/flywheel.