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  1. #1

    Join Date
    Aug 2007
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    Woodstock GA
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    33

    GP1200R Piston Meltdown w/ Pics - Ideas what caused this?

    Motor was rebuilt two years ago but I'd say we put no more than 40 hours on the ski since it was rebuilt. Daughter was out on it this weekend and said it just began to lose power and died. I pulled the plugs out at the lake and hit the starter and a piece of metal flew out of the PTO spark plug hole. Never a good sign when your motor throws things at you....

    So here's what I found when I pulled the head. The #3 piston and cylinder are toast. There were a couple more of those loose metal chunks on top of the piston. The piston is coming apart all around the edge. In one of the pics, you can see a chunk of metal sticking up. All of this has a copper appearance to it.

    I'd guess this cylinder was running a bit hot but I've never seen one come apart around the edge like this. I thought they always burned a whole in the middle of the piston.

    Any ideas out there?

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  2. #2

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    Aug 2007
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    Woodstock GA
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    Would a lean condition at 1/2 - 3/4 throttle cause a piston to melt down around the edges?? Anyone?

  3. #3
    Is this a power valve model? If so is the power valve still in tact and fucntioning or could it be breaking apart and taking the piston with it?

  4. #4

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    Aug 2007
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    Woodstock GA
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    33
    It is a power valve and it has not made contact with the piston. It looks like the piston is simply coming apart. I'm almost certain that it is due to a lean condition but I've always seen them melt down on the center of the piston. What could have caused this to melt down around the edges?? This ski did always have a "pop pop" about mid throttle when just cruising around. Was that a tale-tale sign of it running lean?

  5. #5

    Join Date
    Aug 2007
    Location
    Woodstock GA
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    Anyone????

  6. #6

    Join Date
    Jul 2005
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    Central North Carolina
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    From the pictures thats detnation from too low of octain fuel for the compression and timing you are running! Did you have any old fuel from last year that you used? Only other thing that will do that is crankshaft bearings starting to fail and small bearing peices are comming up and getting on top of the piston! Tommy Jordan

  7. #7
    Can you go deeper into this octane suitable for compression. I have a 1200 xl power valve with 105 compression. Do I need to up the octane above 87.

    My ski has a similar put put about it and I'm trying to figure out the issue before I end up here.

  8. #8

    Join Date
    Aug 2007
    Location
    Woodstock GA
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    33
    The gas tank was practically empty when I started riding this year. This happened on the 2nd/3rd trip out this season so I don't think it was due to old gas. As far as the octane, I've always run 87 in my skis and never had an issue. This ski has no engine mods other than the usual d-plate, chip, clips, etc.

    Don't think it is a bearing because the piston is actually peeling apart. That pic showing the little piece sticking up is actually part of the piston that was/is peeling away. It is still very much attached to the piston. So something is causing the piston to fall apart. I just need to find out what it is so I can remedy that problem before reworking this cylinder. HELP!!!

  9. #9
    Moderated
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    Oct 2006
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    3,711
    Quote Originally Posted by shogan View Post
    Can you go deeper into this octane suitable for compression. I have a 1200 xl power valve with 105 compression. Do I need to up the octane above 87.

    My ski has a similar put put about it and I'm trying to figure out the issue before I end up here.

    you only need to run higher octane fuel when you increase head compression ( to 130psi+) or you advance the timing with an after market cdi such as an advent t3 system or keyway ( i am talking in 2 stroke terms)

    I ran 95 octane with a 140psi head with timing and had no issues, i then ported the motor and increased head comp to 155psi with the same advanced timing and moved up to 98 rated fuel to compensate for the additional compression, this resulted in in more rpm's + 30 or so and reduced my risk of detonation.

    the easiest way to think about it is Octane simply retards the burn rate of fuel , i.e. helps it not ignite before it should due to high compression/pressure .....it quite simply retards this urge to ignite until a spark is applied. when deto occurs the fuel ignites before the piston has completed its stroke and is ready................... the higher the octane rating , the more compression it can with stand. therefore the higher the compressing/timing the higher rated fuel you need......


    at least that my understanding of it.

    ps: i talk in aussie fuel terms as thats where i live but as i understand it US rated 93 is close to our 98 rated fuel.

  10. #10

    Join Date
    Aug 2007
    Location
    Woodstock GA
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    A crank seal leak wouldn't cause this on one cylinder would it? I don't know what a seal leak would do to a motor. Still looking for answers.....

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