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  1. #1

    Hole in Center Piston, What Went Wrong?

    So we restored my '93 SL750 back to a stock '94 SL750 . Ski was running great, did the normal break-in inspected the plugs and everything seemed normal. Upgraded to a triple outlet fuel pump as well. Took my ski all the way from Ohio to Canada and got some runs in up at the summer home and that's where the problem started...

    Was doing some WOT runs and all of a sudden I lost the majority of power and was able to putt back in to the dock at around 20 MPH and that's when I uncovered the issue. Ran a compression test on all cylinders and the MAG and PTO were still firing at 120 PSI but the center cylinder was completely dead...no compression. Took the head off the cylinder and saw that there was a hole burned in the piston! Still unsure of what caused the hole so I was finally able to get some pictures of all the pistons so hopefully you guys can help me diagnose the issue.

    The only known issue is that we forgot to put the gasket between the exhaust manifold and the exhaust pipe. So here are the pictures...if anyone can offer me some insight into what may have caused the hole I would be really appreciative. Hopefully just a carb adjustment but it could be something else. If there is any more information you need about the ski just let me know and I'll try to answer to the best of my knowledge.

    Thanks in advance!
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  2. #2
    ph2ocraft's Avatar
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    Running lean or possible crank out of index.
    What else did you do with the pump upgrade?
    Fuel selector switch?
    New Fuel Line?
    Return Restrictor?

  3. #3
    Checked the fuel selector switch (cleaned it out, made sure it wassending fuel properly). Ran all new fuel lines as well. We did not put in a fuel restrictor but were told it was not necessary on this model. Also checked the crank index the "hillbilly" way but everything seemed fine.

  4. #4
    ph2ocraft's Avatar
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    I think your problem could be no restrictor.
    You need to make sure you have fuel pressure within the system or you will not allow proper fuel supply.
    Is the pulse line to the fuel pump less than 12 inches?
    I'd get a fuel pressure gauge and install it between the carb rack and restrictor to verify at least 2 pounds while idling and minimum 4 at WOT.

  5. #5
    So I should install a restrictor in the return line? And if so, where do I get a fuel restrictor and where does it get installed in the line?

  6. #6
    ph2ocraft's Avatar
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    Are all the tabs on the head gaskets facing or under the water bar?

  7. #7
    Not quite sure...how should all the head gaskets look/face?

  8. #8
    ph2ocraft's Avatar
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    You should be ableto read UP, which I can see in the photo's.
    Then you should have a little "tab" on the head gasket and that should be under the cooling bar.
    Can you get a little better photo of the head gaskets and take all pictures from the same side of the boat and include the cylinder position of each photo. MAG/CEN/PTO, it helps us to know which cylinder is in what condition.

  9. #9
    Yes I'll get those pics as soon as I can.

  10. #10
    Here are some more pictures of the pistons.

    The shot with all of the cylinders was taken from the port side. All individual shots were taken from the back. It looks like all gaskets are on the same way...
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