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Hole in # 1 piston with 50 hours on rebuild

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  • Hole in # 1 piston with 50 hours on rebuild

    In spring I rebuilt my 1997 780 SLT engine, upper and lower. A rebuilt crank from SBT, and I rebuilt my carbs. I pressure tested the engine and did the Pop off correctly for the carbs. Use quality oil as 40:1 pre-mix. No inject

    At WOT it ran great, then would start to loose power, I would back off the throtle and it would start to rev full again. So it really seems like it is starving for gas.

    Then yesterday, after only 20 minutes on the lake, it lost all power and only went to 3000 rpm. At home did compresion test, 3 good, 2 not as good, 1 dead.

    Removed head, hole in #1 piston. Spark plug does not look like it has been running lean, but may be due to trying to run it for 10 minutes with hole in piston.

    I now want to add a second fuel pump to supply #1, the post state that # 1 starves for fuel on these PWC's, even though each carb has it's own line going to it. The post state that #1 gets the left overs from 3 and 2. Not clear on that, like I said, there is a line going to each carb, so can someone tell me why they think #1 gets the left overs.

    I want to put this back together with a new top end and the second pump, but I also read on threads that to check the crank phase to put #1 at TDC and the other two should be in the exact same place. Is this correct? my #3 is about 3/8 inch lower than #2. Is my crank out of phase? This is a rebuilt crank from SBT, told me it was welded so it could not do this, and I do not wave jump. I read it is what twist the cranks from the sudden hit on the water really toques the engine.

    I do not want to do a top end on this engine if the crank is out of phase, am I doing this right to check it this way?

  • #2
    Re: Hole in # 1 piston with 50 hours on rebuild

    Water hydrolocking is the most common reason they get out of phase, moreso than jumping waves.

    When you weld a crankshaft, you weld the rod pins to the counterbalance wheels. You cannot weld the center crankpins. So, although welding can keep the rod pins from twisting on the counterbalance wheels, it cannot stop the counterbalance wheels from rotating on the center of the crankshaft. So even a welded crankshaft can twist under the right circumstances.
    Bill O'Neal <br>
    WCM
    <a href="http://www.watercraftmagic.com"

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    • #3
      Re: Hole in # 1 piston with 50 hours on rebuild

      So by what I described as #3 3/8 inch lower, it is now out of phase? and what is water hydolocking and what causes it? I have never submerged the engine.

      Is this engine worth a top end rebuild or is the crank destroyed? I also hear that a hole in the piston contaminates the bearings. I would think it would just spit it out the pipe.

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      • #4
        Re: Hole in # 1 piston with 50 hours on rebuild

        Why would aluminum getting blown into the crankcases, get spit out the exhaust port? No, it goes onto the lower rod bearing and into the crankcase.

        You need to replace the crankshaft.

        Hydrolocking is when an amount of water or oil or gasoline in the combustion chamber is in a volume larger than the combustion chamber volume, thus locking the engine from completing a 360 degree rotation ( unless the crankpin spins and allows the crankshaft to rotate).

        Anytime you press the starter button and the engine just locks up and does not turn over, you should immediately remove the sparkplugs and turn the engine over with the starter before you re-install the sparkplugs. If you depress the start button again and again, you will bend a rod or twist the crankshaft. Another senerio is when you are riding a ski with water in the hull and you hit a wake and the water bounces up and gets sucked into the sparkarresstor and eventually into the clyinder(s). It can jam the piston from going up all the way and twist the crankshaft or bend a rod, or both. These are all caused by hydrolocking the clyinder(s). Broken reed petals are also common with hydrolocked engines when the cases are full of water too.

        You do not necessarily have to "submerge" an engine to hydrolock it.
        Bill O'Neal <br>
        WCM
        <a href="http://www.watercraftmagic.com"

        Comment


        • #5
          Re: Hole in # 1 piston with 50 hours on rebuild

          The 780's have the three outlet fuel pump so that is not why it burn't a hole in #1.

          Pressure test the engine and see if the front crank seal is leaking.

          Since you need a crank and top end jut get a new block from SBT.
          Providing mobile JetSki service to the Charlotte / Lake Wylie area.

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