30 merc rough running after overheat

I just bought a boat with a 3.

I just bought a boat with a 3.0 merc i/o. I took it out for the first time and didn't notice that it the impeller was bad and overheated the motor right away. It did smoke a bunch even though I was able to notice spark knock and shut it down right away. I have since replaced the impeller and now the motor runs really rough even at idle. I checked spark and am going to replace the fuel filters but am wondering if something may have melted due to the overheat. I checked the inside exhaust bellows and they seem ok but a little melted. Any help would be appreciated.

I would check compression numb

I would check compression numbers first. That will tell the story if you have major damage.

"I would do a vacuum check at

"I would do a vacuum check at the same time. Those two tests will let you know the general health of the motor. I suspect that at least one valve is toast and that will give you the rough idle, or a wiring harness melted to the motor, which is common."

"Blown head gasket. DON'T

"Blown head gasket. DON'T run it anymore until you find out or you could fire-gouge the block, ruining it.

Jeff"