NOt real sure!!! I had an engine compartment deck leak where two MC were mounted in boat. Water dripped down carb into block. I almost pulled motor, but then noticed a small amount of rust on carb..
Lots of unknown variables. Carbureted? Does choke work? Electric or manual??? Has it electric or manual fuel pump? Does MC throttle have switch to keep motor from starting in gear? Does it work.
Check blocked air vent on side of boat. Mud dobbers and corrosion love those small screens. I pulled screen out many years ago that had green crap on it. I had a similar problem as you described.
Lots of unknowns...Will you need access to tank area again??? Is the deck panel you removed all glass or mixture of wood? Will screws be covered with something removable like I/O carpet so boaters will not see them? If it were me, I'd counter sink screw hole, coat screws with 5200 or silicon...
I had a compression tester malfunction on me one time. Get a second tester and see if readings are the same. If so I expect you will have to repower. If salt used, replace block.
If by "reseating," you meant adjusted volves then do compression test to pinpoint area. Examine plugs. If rust or water, you may have to tear motor apart and buy new exhaust manifolds or risers. However, you may have spun a bearing.
Tell him you will sign contract that also allows you to drill 3-4 1/4" holes on the inside of transom near transom assembly, and if crud or rot come out, the sale is off. He is selling the "find" for a reason. I have rebuilt 2 transoms on I/Os. It is a job and requires lots of time and patients.
How used???? Saltwater used? Anti-freeze cooled or only sea of fresh water cooled? When were riser/Exhaust manifolds replaced? If salt used and over 5 years used in salt and totally sea water cooled, expect another 5 hundred bucks or more to replace them. If motor is over 5 years old and used...
Recheck plug firing order. MC have a motor anchor process that makes it different to see motor vibrate, if a plug wire is dead or wrong on plug. Recheck firing order and examine plugs were not fouled or electrode damaged when installed.
