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Looks like that engine was thrown together by the last owner. Base gasket
looked horrible.

You need to pressure test the crankcase before you complete the engine.
Good possibility that there is a lot of porosity in the welds. The case may
not be flat too.

Detonation destroys, it cracks pistons, destroys ring lands and seizes
pistons. Preignition melts. But no melting. Detonation on the rear cylinder
is often caused by a heavy load on the engine that twists the crank and
advances the ignition timing.

The pistons were probably really cheap poor quality pistons too.

You need to inspect the crank too and inspect the impeller too (pitch
and brand).

Lots of work ahead. But a warning, if the cause is not found, it will
happen again.


Bill M.
 

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Many Yamaha triples continue to run well on only 2 cylinders, and
so their owners continue riding them until they melt down completely.

Either that or they are not going to let a little power loss ruin their
weekend. Many watercraft engines are seized repeatedly 5 to 10
times or more and don't stop riding until the engine finally refuses
to restart.


Bill M.
 
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