What happened to my piston? Pics Inside

[geoffw]

Hi Everyone,

My name is Geoff, I just bought a '95 Seadoo Speedster with Twin 657Xs. Its my first boat, however, Iam an auto mechanic so it shouldnt be too difficult to get her running again.

I did a compression test and Engine #1 has 140psi on both cyls, Engine #2 has 30psi on the MAG side and 135psi on the PTO side. Iam at 5500ft of Air so the numbers will be alittle low.

Anyhow, I took the head off and have posted a few pictures to see if you guys can help me figure out the cause of death for my piston. It obviously has a broken crown/ring land...but im not sure why...havent worked on a 2 stroke in years.

Im planning on purchasing a new top-end kit this week.

Thanks for any and all help!
-Geoff

 

[DonoBBD]

Detonation will break the ring lands off just like that. Skidoo had big problems with pistons do just this in the 800s and hypereutectic pistons. They broke off at the weak point right where the top rings pin is. This can be from hours of use and poor gas.

Best fix is to replace all the pistons in both motors because this is a wear part and will fail on another. I have seen this with the Cat small motors where the ring positioning pin comes right out. This is why that pin is not on the exhaust side when putting the pistons in.

Don.

 

[parrothd]

I'd rebuild the entire fuel system before anything....

 

[Dr Honda]

Piston Gnomes were in there eating at the edges. (That's the most tender part)


Sorry....


Umm.... well... it's melted. So, it was hot.... I would say someone was running it with a plugged up carb. It ran lean, and ate itself. But the strange thing is... it normally shows up on the exhaust port first. (Where the heat is concentrated)

 

[Dr Honda]

Am I seeing other ridges in the piston crown, around the melted spot? If I am... I'm going to say there is a defect in the piston too.

 

[geoffw]

I removed and pulled the engine apart, the jugs are at the machine shop now and I plan to split the cases to inspect the crank and replace the seals.

Also, Ive just ordered 4 carb rebuild kits, bought 8 new plugs, drained all the old gas and will be replacing all plastic oil & fuel lines with automotive grade rubber.......Hopefully she'll be reliable!

Ive also swapped in a Group 34 Marine Battery and ditched those junky motorcycle batteries.

Can anyone confirm that the oil injection system is gravity fed and there are no electric in-tank motors to deal with?
Also, should I rebuild the oil pumps?

Thanks for all your helps guys!

-Geoff

 

[parrothd]

Since you're in the carbs replace the needle and seat, those wear out. Oil injection is gravity feed. You should also, change the pump oil and inspect replace the wear ring and props(repaired) if needed.

 

[geoffw]

UPDATE:

I decided to inspect the bottom-end so I split the cases and found both bearings on the MAG side of the crank were shot...My best guess is that the shot bearings allowed the crank to move around which killed the crank oil seal and let in air which leaned out the cylinder and melted the piston.

Since this engine was the original engine for the boat and has 15+ years of abuse on it, Im guessing it had at least 400-500hours on it, so it was time for a replacement anyhow. Hopefully the other engine doesnt let go anytime soon.

On another note, both Jets look perfect. Wear Rings have no visible or physical ridges, divets, etc. Impellers have sharp edges and by no physical damage. By the looks of everything is looks as if it was a well maintained boat.