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Discussion Starter · #1 ·
Looking for some advice on a rebuild.

My '88 650sx overheated late last summer. Engine got really hot.

I lost some compression from the overheat incident. There was enough compression to run, but it didn't want to start or idle as well. It was approaching the end of the warm season here in Minnesota, so I fixed the cooling issue, ran the ski around, and when I jumped a wave I heard a distinct sound of what sounded like a piston ring broke free, and I limped the ski back to shore at full throttle. It ran very poorly, but got me home. The end result was the engine lost pretty much all compression, but I can't recall if it was only one cylinder or both.

I tore into the top half of the motor and I find no obvious internal damage of any kind. All the top end engine seals were damaged from the heat, and it was a struggle to get the top end apart, but with some careful work I got it apart.

Pistons, rings, everything looked fine.

Curious why the ski made an obvious, brief, loud, sound of metal on metal before it went almost dead. Trying to figure out what failed because I expected the entire top end to have damage.

Any advice is appreciated.
 

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Maybe the sound was in the bottom end of the motor? Without having to pull the motor you can check rod bearings by rotating the crank to top of the stroke and right when its about to goes down you can push on it and if it falls some you have a rod bearing issue. Could be something different but that’s something easy to check.
 

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Discussion Starter · #3 ·
Not a rod bearing issue.
The sound I heard was quick, and it was gone. The engine never made another sound that something was wrong.
I took the reed valves out and did not find anything wrong.
The prior owner rebuilt the bottom half of engine with all new.
 
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