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  1. #1

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    slx 780 hole in piston

    My buddy has a 96 slx 780 that keeps burning holes in the mag side piston.This is the 2nd time its done it.We have checked the jets main 110 slow 67.5.We had the high speed screw at 3 turns out.We still cant figure whats going on.If you have any suggestions please let me know.Thanks












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  2. #2
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    Have the carbs been cleaned/rebuilt? Leak down test done? Lots of things could cause that.

  3. #3
    Polarisitis loonatik's Avatar
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    Pop off pressure may be too high on the Mag carb or Mag cylinder may have an air leak.
    Check fuel pump pressure too.

  4. #4
    urugol's Avatar
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    Quote Originally Posted by barge bait View Post
    My buddy has a 96 slx 780 that keeps burning holes in the mag side piston.This is the 2nd time its done it.We have checked the jets main 110 slow 67.5.We had the high speed screw at 3 turns out.We still cant figure whats going on.If you have any suggestions please let me know
    Have you rebuilt the carbs as well? I would go right through them, check your needle and seat assemblies, spinrg, pop off pressure also. Are you running premix (what ratio) or oil injection?

  5. #5

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    Quote Originally Posted by urugol View Post
    Have you rebuilt the carbs as well? I would go right through them, check your needle and seat assemblies, spinrg, pop off pressure also. Are you running premix (what ratio) or oil injection?
    He is premixng 40 to 1 and a little more.And we rebuilt carbs last year.Why would the pop off pressure be an issue thought that was for the low end?

  6. #6

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    Clean and rebuild the carbs, set pop off pressure, install a triple fuel pump from Mikuni, run the 3 inlet fuel lines to each carb also a lot of people over look the fuel filter inside the carbs replace it a new one should come with rebuild kits Use only OEM Mikuni kits.
    Also check the pulse lines for damage you could have a small hole.
    Aslo if your runnin premix you may have a air leak at oil injector line where it goes to the intake system where you block off when doing the block off kit.
    You have a lean condition you could have a bad front crank seal also and one other thing that I have found is the thermostat Replace it.
    There are so many things that can cause this along with Ignition if it was Mid or rear cylinder Id say twisted crank but not here.

  7. #7
    Resident Coffee Addict Oshawapilot's Avatar
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    Are the 780 equipped ski's not the ones with the stock single-outlet fuel pump that MUST be upgraded to the three-outlet?

    I know when I was contemplating a 780 equipped ski here a few months back I was warned left right and center that if I didn't upgrade the pump this was often the result.

    (Edit: I see someone else beat me to it.. )

  8. #8
    BlueFishCrisis's Avatar
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    Quote Originally Posted by Oshawapilot View Post
    Are the 780 equipped ski's not the ones with the stock single-outlet fuel pump that MUST be upgraded to the three-outlet?

    I know when I was contemplating a 780 equipped ski here a few months back I was warned left right and center that if I didn't upgrade the pump this was often the result.

    (Edit: I see someone else beat me to it.. )
    The 780's should have come with a pentagon style fuel pump which already is a 3 outlet with plenty of fuel flow. They should, however, be rebuilt after 16 years to ensure proper operation, along with all of the other above suggestions......

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