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I have a 1992 40 HP Mercury four cylinder that quit on me. I had a trusted mechanic do a compression test and it came out 110, 95, 125 and 115. He said the engine is not worth rebuilding. He also found a leak in the oil line so it more than likely ran without oil being injected. I have this engine on an old but pretty nice BassTracker. What are my options? What is the cost of a rebuild or should I junk the entire boat or try to find another motor to go on it?
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Rebuilding is expensive on a 4 cylinder engine it could be anywhere from 1000 to 4000 dollars. It would depend on what is damaged in the engine. -
Those values seem not too bad yet.(Minor Scoring). I would suspect scoring is due to carbon deposits or possibly an overheat. This can be verified by removing the exhaust plate on the port side of the engine. This will gain access to the pistons and cyl walls. If there is damage all around the piston, lack of oil or an over heat has occured if it just on the exhaust side, carbon or poor combustion is to blame.
A decent honing of the engine and new rings may be all you need.
Lack of oil will generally show up on #1 cyl first as it runs the hottest making it far less tolerant of low oil.
To check for a broken ring once inside the exhaust cover, press lightly against the rings on the pistons. If the spring, they are fine and not broken. But given the compression values..they should be replaced.
But to be on the safe side, check the crank and all brgs for hot spots or dark area's once the engine is apart.
The main repairs can often be done by someone with a decent Factory Shop manual and some tools.
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