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  1. #1

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    Angry 94 slt750 hole in piston

    I have a 94 slt750. It keeps putting a hole in the piston, that is closest to the front of the boat. After being fixed it runs well for a couple of hours then loses power then dies. The mechanic has checked the crank seals, carbs, and it has the fuel pump upgrade, all ok. So he says.The plugs when pulled: front one is clean, back 2 are black if not even a little oily.The timing when checked was off and mechanic adjusted the stator. Crank has not been replaced. Compression is good on back 2 pistons. This situation has happened 3 times. What to do?


  2. #2
    axgrider73's Avatar
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    Welcome to the Green Hulk Forums!!!

    Have you checked the crank index?

    Have you checked the temperature of that cylinder to see if it is running hot?

  3. #3
    race24x's Avatar
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    Isnt a hole int he middle top of piston caused by detonation? If it is lean do you have the three output fuel pump? I have read alot on here about that and it seems like it fits this scenario. A weak fuel pump with one inlet will not have anough fuel to supply the Mag cylinder with enough fuel. Just a guess though. Welcome the guys on here are great and will likely give you more thelp than I can

  4. #4
    ph2ocraft's Avatar
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    Welcome to the Green Hulk Forums.

    What are your max RPM's?
    Assuming the fuel system IS good.
    I'd also say the first thing you need to do is verify crank index so you're not chasing your tail.

    An unexplained holing of one cylinder is a primo example of bad crank index.

  5. #5
    SPEED KILLS, BUT YOU GET THERE QUICKER Keddano's Avatar
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    What did your guy do to check the Crankseals,Did he say he did a leakdown test?Maybe you should do it yourself.Also what # spark plugs are you running?

  6. #6
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Have you changed the fuel lines? If so, did you reinstall the fuel restrictor if it had one? Have you cleaned the carbs at all?

  7. #7

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    The 3rd time it died I pulled the plugs and had a look before I dropped it back to the mechanic. The piston that has been blowing seemed fine, nice toasty brown plug. The very last plug towards the back of the boat, the electrode was crunched. I put a flashlight down and you can see where the piston hit the plug. As for crank index I have no idea if this was checked. I will bring it up. No fuel lines have been replaced. He said that there has been a secondary fuel pump added. I am not sure if there is a 3 output pump installed or not. A leak down test was done. Plus when the timing was adjusted via the stator a visual was also done. I will be bringing up all of these questions with the mechanic. thanks for the added info.

  8. #8
    SPEED KILLS, BUT YOU GET THERE QUICKER Keddano's Avatar
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    If the piston top hit the piston,1 of 3 things has happen,1. The wrong plug was installed,2. the crank bearing or the wrist pin bearing on the rod has gone bad.3. the rod has broke and allowed the piston to slap the plug.That is the only way

  9. #9
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Or something got sucked into the combustion chamber and crushed it.

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