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  1. #1

    why is my 99 polaris pro 785 blown up? look at these pics

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    I have this 99 polaris pro 785 I am working on and I am rebuilding the top end.

    I am trying to figure out why it is blown up

    I am just wondering what the problem looks like to you guys. the middle and rear cylinders had good compression and look great. it is only the front cylinder that is damaged

    Any Input would be greatly appreciated


  2. #2
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    That's not blown up... That's severe scuffing.

    1. What make of piston are those? OEM, Wiseco, other?

    2. What are the piston clearances set at?

    3. Are you running premix or using the oil injection?

    4. What does the other side of that piston look like?

    5. Did you overheat that cylinder?

    6. Were you running that engine on the garden hose for an extended amount of time?

  3. #3
    Why would you not call it blown up? The piston and cylinder were severely damaged, it had 0 compression on that cylinder, and the ski would obviously not run. I know the rod isn't bent or broke, the cases are fine and it didn't break the piston into pieces but in my book it is still "blown up"

    Anyway it was OEM Polaris pistons

    The piston clearance before with those pistons I am not sure what they had it at. This ski was rebuilt by a different shop then it blew up on the owner then they decided to sell it and it was bought by a local marina that sends me jet ski work. Now I have it and I rebuilt the top end with a Wiseco Piston kit. the piston to cylinder wall clearance is now .005" which is right in spec.

    it is oil injected but I am converting it to premix

    as far as overheating and/or running it too long on a hose I am not sure I am not the one who had it when it blew up. I can't talk to the previous owner because like I said it was bought at an auction by a marina then brought to me.

    I have pictures on my 1st post of both sides of the piston. then the 3rd pic is of all 3 pistons. I didn't show the other side of the other 2 pistons because they are in perfect shape.

  4. #4
    johnsonmtz's Avatar
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    All three of those pistons look solid black on the top. I would be inclined to say this is the result of a lean condition.

    KJ

  5. #5
    lxman88's Avatar
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    yea the piston wash on all of them dont look like they were tuned in right

  6. #6
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    I wouldn't call that "blown up" because it's still intact. Anyways......

    There are 2 large gouges running the length of the piston. Were all the wrist pin bearings in place when you removed the piston?

    I'm wondering if a bearing broke free and the piston was "flopping around" inside the cylinder?

    Otherwise, my guess is lack of lubrication. Is/was the oil hose in good shape and connected on that cyl's carb?



    Be careful with those Wiseco pistons. On the older batches, they used to "cold seize" if the engine wasn't properly warmed up. Many Polaris guys went as far as 6 1/2-7 thou on the clearance to prevent that from happening. It seems to me that the newer batches are made a bit heavier, my thinking is more material to slow the rate of expansion to combat that problem.

  7. #7
    yea all of the piston pins circlips and bearings were in place when I removed the piston.

    the oil lines all look good. but like I said I think I am going to switch it to premix. I am going to clean the carbs and check pop off. maybe I will richen it up a little.

    I will be sure to warm it up properly and make sure the marina and/or whoever buys it knows this and its history as well.

    I appreciate your help

  8. #8
    lxman88's Avatar
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    the oil injection is not bad on the polaris skis as long as its maintained.

  9. #9
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Right ^^^^^^^

    What normally happens though, is the hoses get hard and crack off the carb. Then bye bye piston. . . .

  10. #10
    Yes I have seen that many many times!!! I try to tell customers about maintenance and what to look for but they never listen. o'well if they all listened then I would be out of a Job.

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