Is this the 3.7 litre merc? They had real issues with the exhaust manifold rotting out--I have seen this several times with these motors. Start at the basics before going crazy. Pull the exhaust manifold and riser and inspect them--I will bet they are the source of your problem. Hopefully you did not hydraulic a rod. These are expensive engines to rebuild if it comes to that. A 150hp outboard will out run and out gun that 170ho I/O.
Good luck with the issue and your decision.
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 88 V-20
200 G2
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