Blown piston need diagnostic help
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  1. #1
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    Question Blown piston need diagnostic help

    Hey guys, I have a 1999 tigershark 770L. When I got the ski this year it had a blown piston (which I was told was do to being run with no oil) so I rebuilt the top end(2 new pistons) and had it running on the water for about 5 hrs before it blew another piston. I had run the machine with pre mix 50:1 so I am positive this time it was not caused by lack of oil. The one piston still looks brand new and in good condition. Iím wondering what could have caused this. Could it be something as simple as a spark plug came loose and air was being drawn in or could I possibly have a small crack somewhere and if so would this cause the piston to blow? Does anyone know how I can check for small cracks and if they are easily fixable. Thanks.

  2. #2
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  3. #3
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    Re: Blown piston need diagnostic help

    Most likely an air leak on that side leaning it down.

  4. #4
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    Re: Blown piston need diagnostic help

    Where is the air most likely coming from? I plan on pulling the rest of the motor out tonight.

  5. #5
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    Re: Blown piston need diagnostic help

    I would say the crank seals. But could be the intake, bottom of the cylinder, etc. If your handy could make some block of plates and pressure test it. Might get lucky and pin point it. I doubt it was the plug unless it was loose when you checked it. Even then I would doubt it. Hard to get a good read from the piston s but I doubt it is the carb given it is a single and it occurred on the same piston. Got to make sure surfaces are flat. Idiots have a habit of taking stuff apart with a hammer and screw driver.

  6. #6
    PWCToday.com Is My Home Away From Home WFO Speedracer's Avatar
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    Re: Blown piston need diagnostic help

    I went through this years ago with an SBT swap, customer brought me the engine and wanted it put in, I water tested it and it ran out fine . Now of course it was a brand new engine running with premix and you couldn't open it up wide open. A few weeks later it's back, top end is dead again, I pulled the engine and send it back to SBT , new engine come in and we repeat the process. A few months later it comes back, top end is dead again,

    As it turns out somewhere down the line there was a major recall, the customer never got the recall letter, a bunch of 770 engines had apparently gotten loose from the factory with the wrong main jets installed in the carbs, of course the premix in the fuel tank only aggravated the issue and once the first two tanks of fuel were gone and the engine got opened up it seized due to running too lean.

    Just something you might want to look at before wasting a lot of time , effort and money.
    Y'all know me, still the same O.G. but I been low-key
    hated on by most these &^$$@s with no cheese, no deals and no G's, no wheels and no keys, no boats, no snowmobiles, and no skis, mad at me cause I can finally afford to provide my family with groceries

    WWSRD , What would Speed Racer do ?

  7. #7
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    Re: Blown piston need diagnostic help

    Thanks for your fast replies. Ok I will take all of this into consideration.
    As for the recall that you are talking about. Would this happen even when the ski was pre mixed or only after they stopped running it with pre mix. I had the ski running premixed when it blew up (50:1). Cause I was thinking of just buying a brand new motor from SBT since I got the jet ski for free, and everything else is in very good condition. Is there anyway to tell if I have the correct main jets in my carb and do they still affect a lean condition even if I plan to always run the ski premixed?

  8. #8
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    Re: Blown piston need diagnostic help

    Quote Originally Posted by Justinmckain View Post
    ....Cause I was thinking of just buying a brand new motor from SBT since I got the jet ski for free, and everything else is in very good condition....
    I will NEVER buy another SBT engine. You need to carefully disassemble your engine. Inspect everything, decide what parts you need to assemble it correctly. People here will help you. I have two 1998 L's. They are great little boats.

    Running premix will cause your engine to run leaner. I bumped up my jets a size when I switched to premix.

    As mentioned above, if you have not yet taken it apart you need to do a pressure check on the engine. There are descriptions here for how to do it.

    I have a 1999 service manual if you need pages scanned, let me know. According to the manual your '99 L should have one BM 40-38 carb, low speed jet-70, high speed jet-147.5, High speed needle valve is fixed - 150, low speed needle 1 turn. These sizes are for oil injected stock engine.
    2, 1998 TS 770 L, restored and running great as of July 2019 (NO THANKS to the person at SBT, who put the WRONG crankshaft bearing into the rebuilt engine I got from them)

  9. #9
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    Re: Blown piston need diagnostic help

    You'll have to see if WFO or someone else can give you jet specs. What he is trying to tell you is that switching from oil injection to pemix will lean out the jetting. This combined with the already lean jet do to go mistake and recall results in disasater. It is leaner because the oil that was once injected now travels though the jets displacing a proportional amount of fuel.

  10. #10
    PWCToday.com Is My Home Away From Home WFO Speedracer's Avatar
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    Re: Blown piston need diagnostic help

    I do not have the jetting specs. after all the issues with the ski the customer sold it and moved on
    Y'all know me, still the same O.G. but I been low-key
    hated on by most these &^$$@s with no cheese, no deals and no G's, no wheels and no keys, no boats, no snowmobiles, and no skis, mad at me cause I can finally afford to provide my family with groceries

    WWSRD , What would Speed Racer do ?

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