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  1. #1
    97 Kawasaki STX 1100
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    Please Help- Polaris 750 SL

    Ive had this jet ski now for a month and in this time Ive melted 3 PTO pistons. Holes in the center. I started the first repair by replacing the rear out pto seal. Then I melted the pto piston again within 24 hrs. I pulled the motor and replaced the case seals front and rear. 3 seals total. again 24 hrs later I melted another hole in the piston. When the piston holes it creates crank case pressure and pushes out the rear outer pto seal. I tapped it back in pulled the carbs and discovered they had been severly ported. The choke had been removed etc. I decided to buy a new set of carbs and rebuild them. I put the ski together again with all new intake gaskets and inspected the reed valves and tried my luck again. Left the 5 mph zone and herd engine noise. Pulled over checked the plugs, the rear pto plug looked light brown( not black). Changed the plugs there on the water. Decided since the motor was making noise to take it back out and reinspect my work... Pulled the plugs and this time the rear pto plug was completely fouled oil and gas wet. I checked compression only 80 psi pto cylinder so I removed the head. The top of the piston was wet but no obvious damage so I pulled the jug. Remember less then 5 minutes in the water. The exhaust side of the piston was all scratched up and the piston ring was stuck in the piston. Please someone tell me what the cause of this is? Do I have the same problem Ive been having or a new problem. Is it possible Ive damaged my rear pto seals when I melted that last piston again....


  2. #2
    97 Kawasaki STX 1100
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    If my index was wrong wouldnt my ski run bad? Or misfire? Prier to changing carbs my ski ran great right until it melts the piston. I did take those old carbs apart and find some sludge in the needle and seat on the pto piston. I also did a pop off pressure check on both the new carbs and the old ones so I know the new carbs on now are working correctly.

  3. #3
    TopCop931's Avatar
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    If the old carbs had sludge in them that may have been your original problem for the lean condition. The most recent problem could have been an issue with the rings. I would wait and see what some of the other guys think.

    I see that xlint has already advised you on the crank index on the other thread.

  4. #4
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    I just wan't to clarify something. If you hole a piston, it will not pressurize the case and "pop out" your crank seal. If anything, crankcase pressure drops because the air can now escape past the piston and right out the exhaust port.

    Index the crank to make sure it's not your problem.

    If you had sludge in the needle and seats, it got past the filter somehow. 1. did you clean/replace the filters inside the carbs? 2. did you verify the jets were clear? The sludge may still be clogging the main jet.

    If you're burning holes that fast, I'd suggest you start checking your repairs before going WOT. You can catch an issue before you burn a hole by using a small flashlight through the plug hole.

    Do you have the restrictor installed in the return hose? If so, what size is the orifice?

    Did you properly gap your piston rings? Or did you just install them as is?

    Have you done a leak down test since "popping out" the seal? The inner seal has failed if pressure has bypassed it, and made it's way to the outer seal.

    Finally, have you checked your crank bearings for debris? 3 holed pistons is alot of aluminum to be ground into the crank bearings, and MAY be causing the noises you're now hearing.

    See my response in your other post for other possible causes.

  5. #5
    97 Kawasaki STX 1100
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    Quote Originally Posted by xlint89 View Post
    I just wan't to clarify something. If you hole a piston, it will not pressurize the case and "pop out" your crank seal. If anything, crankcase pressure drops because the air can now escape past the piston and right out the exhaust port.

    Index the crank to make sure it's not your problem.


    If you had sludge in the needle and seats, it got past the filter somehow. 1. did you clean/replace the filters inside the carbs? 2. did you verify the jets were clear? The sludge may still be clogging the main jet.

    If you're burning holes that fast, I'd suggest you start checking your repairs before going WOT. You can catch an issue before you burn a hole by using a small flashlight through the plug hole.

    Do you have the restrictor installed in the return hose? If so, what size is the orifice?

    Did you properly gap your piston rings? Or did you just install them as is?

    Have you done a leak down test since "popping out" the seal? The inner seal has failed if pressure has bypassed it, and made it's way to the outer seal.

    Finally, have you checked your crank bearings for debris? 3 holed pistons is alot of aluminum to be ground into the crank bearings, and MAY be causing the noises you're now hearing.

    See my response in your other post for other possible causes.
    Carbs have been replaced with new oem ones... They have been disassembeled cleaned and put back together. The pop off pressure has been checked too... ( completely different carbs now) All new intake gaskets( reed gaskets, carb gaskets)

    The most recent piston installed was a used over sized piston with the matching jug. I never even rehoned it cause it was clean... Within 5 minutes I had severe scuffs up and down the exhaust side of the piston. What causes this? The top of the piston was clean. This piston has been run before and the walls of the jug were nice and clean. Same piston rings too.... Can this damage be caused within 5 minutes from a lean condition?

    Im guessing I installed the piston rings wrong, have a leak from the rear seal or the index is wrong....

    I will try checking the index today. I dont have a degree wheel. I will first try the redneck way. Center cylinder up and front and rear should be even. I have a dial indicator and a pencil....

  6. #6
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    How about a pic of the latest piston failure?


    Scuffed walls can be from lack of oil, too tight a piston ring clearance, cold seizure, piston clearance too tight, overheat seizure, or EXTREMELY lean condition eroding the psiton on the exh side.


    A clean piston top often means a water leak into the combustion chamber. It actually "steam cleans" the piston top.

    a pic will help determine the cause of the failure.

  7. #7
    97 Kawasaki STX 1100
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    Quote Originally Posted by xlint89 View Post
    How about a pic of the latest piston failure?


    Scuffed walls can be from lack of oil, too tight a piston ring clearance, cold seizure, piston clearance too tight, overheat seizure, or EXTREMELY lean condition eroding the psiton on the exh side.


    A clean piston top often means a water leak into the combustion chamber. It actually "steam cleans" the piston top.

    a pic will help determine the cause of the failure.
    Sorry for the delay... Let me fix my earlier comment... Clean piston on the top! My piston top was very wet of oil and gas. The spark plug was wet.... smelled like fuel. Almost like I had a carb issue. LOL

    I sent you a message earlier so now we know my index is off approx 1/8.

    For everyone one else, I started by checking the index the redneck way. Found TDC center cylinder and checked to see if front and rear cylinders matched height using a screw driver. They didnt. I made a degree wheel , my wifes a graphic designer so she made it. 0-120 and 240 marks and rotated the crank 120 degrees at a time once finding TDC on center cylinder. Cylinders 1 and 2 lined up perfect but cyl 3 PTO cylinder is off by 1/8 or more. Ive removed the engine today and the crank is out.... Cleaned the case and Im ready to put it back together as soon as i find a crank or get mine fixed...

  8. #8
    97 Kawasaki STX 1100
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    Quote Originally Posted by xlint89 View Post
    How about a pic of the latest piston failure?


    Scuffed walls can be from lack of oil, too tight a piston ring clearance, cold seizure, piston clearance too tight, overheat seizure, or EXTREMELY lean condition eroding the psiton on the exh side.


    A clean piston top often means a water leak into the combustion chamber. It actually "steam cleans" the piston top.

    a pic will help determine the cause of the failure.
    The first pic is the most recent piston. This piston is a wiesco forged piston. This accured within 5 minutes of riding the ski. The next pic is the 2 most recent failers. The ski ran good the first day with lots of power. These are STD oem pistons. The third is a top view of the most recent forged piston. Theres a 4th piston too but I through it away after fixing the ski the first time...
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  9. #9
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    That first pic (Wiseco) is seizure damage.

    Second pic is heat related dome damage. (lean, too much timing, wrong heat range spark plug, or out of phase carnk)

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