help me figure out why it blew up!

[mh0384]

I recently bought a 95 sea doo xp 720. it was rebuilt, bored approx 5 running hours prior to me buying it.

i rode it about 10 hours trouble free, until i filled the injection oil with walmart quicksteel? oil for PWC's, 2 stroke outboards. It had blue sea doo oil in it, about half a tank. I topped it off with the greenish walmart oil.

i ran it about 5-8 minutes, and it shut off... restarted and died numerous times after that until it didnt start anymore...

so heres some pics of what it looks like.. the bad side is the intake side of the piston and the exhaust side has the 2 stripes...

maybe the oil was coincidental... maybe it was lean... maybe the oil injection got a clog?

i dont see how it would have run lean since the top of the piston is perfect, even more blackened than the other good piston is

all i know is its been rebuilt twice in 15 (approx) running hours, and i really want to figure out why so it doesnt happen again

i took the carb apart on the cyl that blew and it seemed pretty clean... the filter had a little junk in it but not full or anything

it was the front cylinder that blew, the rear one has 125 psi of compression

the previous owner is 99% sure it was the opposite cylinder that blew last time...

the ski HAD gray fuel lines, i just changed them all... but i cut them apart and they werent real gunky or anything...


thanks for any help! sorry for the long post!!

 

[evanssd1]

same thing happened to my 95 XP 800, pistons looked almost identical

 

[mh0384]

Did u figure out why?

 

[spx800]

im not saying this is why, but you're not supposed to use oil for outboard motors. they're rated tcw-3 and you are suppose to run ashless api-tc or something (i can't remember what), but it says on the sea-doo somewhere. I can't really tell, but your wash should be about a pinky nail in size on the piston where the ports would be.

 

[Dr Honda]

The crown isn't melted... so it was lean, or no oil.


In reading your post... there was a few no-no's.

first... if you have gray hoses... change them, and clean the carbs. One carb could plug up before the other. And... when you go lean, you aren't transporting oil into the engine.


Second... NEVER mix 2-stroke oils. Bad things are known to happen. One of witch, is that they turn into jello. You may want to pull the hose off the bottom of your oil tank, and see if it has any chunks floating around in it.

Third... the oil you used may not be a API-TC oil. If it said outboard engine oil, it was probably a TC-w3 oil. If that is the case, it doesn't provide enough protection for a seadoo engine. If it was QuickSilver PWC, then on the bottle it should say API-TC, and "For use in SeaDoo."


Regardless what happened, I would pull the oil system apart, and check everything.

 

[mh0384]

hmm i agree dr. honda.. it seems too coincidental that it ran for about 10 min after putting that oil in for the first time...

im going to check when i get home to see what type of oil it is.

if it was leaning out would the plugs still be black?

 

[jwick86]

No, if it was leaning out the plugs wouldn't be black. They would be almost white.

About 720's, is the ski all stock? 720's are known to blow up just by putting an after market prop on them. They can also blow up very easily if you put after market flame arrestors on them and do NOT put the carb brace. The carbs will actually bounce enough in the hull to crack the intake manifold and lean out. There are numerous things that these 720 motors do not like.

I would not use any oil that is TC-W3. Not recommended in Seadoo's at all. Seadoo's don't run oil that other brands (Yamaha, kawi) run.

Other idea's, the piston to wall clearance was too tight and changing oils didn't provide enough lubrication inside the cylinders.

Which piston is this seizure on BTW?

Is your timing advanced at all?