Below are pictures taken of a 1993 Wave Runner with stock 650 motor. Ski brought to me with no pump. So reinstalled one.

I do not history of motor other than it had unknown hours on it and don't know how long it had sat unused.

BUT.... motor looked brand new to me. No dirt or any corrosion. Might have been recently installed for all I know.

Started and ran GREAT. Ran all summer with maybe 20+ hours on it. Then stopped running.
No compression on rear cylinder so pulled head. This is what was found.

Oil injection system was removed and blocked off.  Owner running premix with Yamaha Lube at 50:1 and premium gas.

Front piston looks normal to me. Motor has single carb.

 Rear piston has NOT been cleaned.  You are seeing it the way it came out.  One piston skirt is missing and the other skirt is cracked also and just about to come off.  Bad scuffing on side and piston clearly melted on sides and top outer edge.  Looks to me like piston crown is badly pitted with dents but does not look like detonation damage to me.

Not pictured but sparks plugs both cylinders looked normal and were intact.  Rear piston sleeve has minor scouring as expected. 

BUT..... entire head looked normal.  Gaskets looked normal.


OBVIOUS QUESTION..... What do you guys think happened to this?

no oil in rear piston caused the whole problem like you said, and maybe it wasn't 50:1 maybe it was straight gas. just a guess. also you didn't mention if you did a compression check before you got the ski. that would have told you from the door if there was something wrong in there. atleast you got all winter to get her back on her feet.
mike

Mike -

I agree.... my thinking is that something stopped lubricating the rear cylinder. The obvious answer would be no oil in gas. BUT THAT IS NOT IT. Gas was mixed 50/1 with premium and yamaha lube. Have to trust me on this one.

After failure, engine would START and run on front cylinder ih normal oil smoke.

Other easy answer would be rear piston running lean causing detonaion. Overheats and trashes piston in short time. BUT.... piston doesn't show signs of what I would expect of detonation. Center would normally be discolored. And this is single carb motor. AND ... rear crank seal is fine. What would cause rear carb to go lean?

As for compression before failure.... was not a consideration because engine ran perfect (idle tru all rpm).

What else would cause lube loss. Water in piston ?


STUPID QUESTION: Piston overheated obviously. Would this cause the piston skirts to crack off?

KEEP THE IDEAS COMING !!

Yes... I can put back together easy but.... DID I SOLVE THE PROBLEM ??? What IS the problm?

hey, as for the water getting in the cylinder. i filled all 3 of my cylinders up with water when i first got my ski. i put the hose on it and turned the hose on withount turning the engine on first, then i couldn't figure out why it wouldn't start it was hydrolocked. i later learned from a yamaha certified mechanic that had it started it would have bent the rods and cracked pistons and did all kinds of narly stuff. it should be noted that the ski was brand new and all seals were in new condition and water still got into the cylinders.
and your piston skirts cracking off most likely was caused by overheating or the piston slapped on them pretty good.
mike