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  1. #1

    PTO piston burns twice in a row SL 750

    I recently scored a great deal on a 95 SL 750 that had a burnt piston. Honed/reringed the middle, and replaced the cylinder jug, piston and head on the PTO cylinder. While I was in there I ended up installing the 3 outlet pump and new lines to the carbs. Tossed the motor together, checked the compression and fired it up. Started great and ran awesome. I broke it in with the varying speed, ride it like you stole it style. I've thrown car motors together and thrown them on the dyno for the "break in", so I know this method wasn't the issue. It ran great all weekend, we used it to wakeskate, ran out to 50+mph(on the gauge, so take that for what its worth lol, but wasn't a slouch) and accelerated well. I was driving it around a little before I headed home, and it bog on take off, but seemed to clear up once I got it up to speed and didn't seem to be an issue. My buddy took it out and it was still bogging. We pulled out and I checked the compression and sure enough, it was 0. Ripped the head off right there, and it blew up on the same side as it did originally, right on the exhaust. I was premixing on top of the oil injection during break in, so I'm about 99% sure it wasn't oil related. I've been searching a lot about the problems previously, but if it were you, what would you place money on it being? It this about pretty much guaranteed to be a carb issue?


    tl;dr PTO piston melted twice on exhaust side, once on relatively fresh motor while premixing, upgraded pump and new lines. Is your money on a carb issue? Thanks in advance for your help!


  2. #2
    I'm addicted to Polaris PWC ghostinstallations's Avatar
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    Post a pic of the top of the piston. My guess is the carbs need to be rebuilt or tuned. This is a lean issue, not an oil issue.

    Were the carbs rebuilt using genuine parts?
    Check fuel pressure after 3 outlet pump installed?
    Checked piston wash at all?

  3. #3
    MPower's Avatar
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    Always need to determine the reason it toasted the piston the first time, otherwise it will keep happening. Check the carbs and pressure test the motor.

  4. #4
    Quote Originally Posted by ghostinstallations View Post
    Post a pic of the top of the piston. My guess is the carbs need to be rebuilt or tuned. This is a lean issue, not an oil issue.

    Were the carbs rebuilt using genuine parts?
    Check fuel pressure after 3 outlet pump installed?
    Checked piston wash at all?
    Sorry, I didn't snap a pic and it's at my cabin, but I'll be back there Wednesday. Thinking back on it, it definitely looked lean...that greyish look. Total oversight on my part on this deal, just needed to bounce it off a few other people, I assumed it was fuel related. I'll admit, I'm very newbish when it comes to carbs, I've only really built/worked on EFI cars so it's a bit of a foreign concept to me. I'll pull them this week and take a look at them.

    Quote Originally Posted by MPower View Post
    Always need to determine the reason it toasted the piston the first time, otherwise it will keep happening. Check the carbs and pressure test the motor.
    Yeah, oversight by me. I read all the issues with the fuel pump I sort of narrowed in on that being the problem overlooking the fact it was a mag cylinder issue. My mistake 100%, but they are fairly simple to take a part so I'm not TOO concerned. I'm just glad I didn't sell it and dump it on some poor unsuspecting person - not my style.

    Thanks for the help! It's always nice to be able to talk things through with others more experienced than myself.

  5. #5
    Just a quick question after looking over some stuff, I see when you remove the autocock, you are supposed to cap that extra line. Well...I noticed mine is laying uncapped in the bottom of the hull. Would that have ANYTHING to do with this issue? Or is it completely unrelated? Obviously I plan on capping it off anyway, just trying to get to the bottom of the issue. And if nothing else, someone like me can find this answer here later...I know I'm often here after googling to death.

  6. #6
    I'm addicted to Polaris PWC ghostinstallations's Avatar
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    I assume the line in question is a vac line from the bottom of the crankcase. If so, It likley has alot to do with your issue. That will lean out the motor, and the first piston to go would be the PTO as that is the section of the crank where the vac line attaches to.

  7. #7
    BlueFishCrisis's Avatar
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    I believe the Autocock was operated via a vacuum line on a fitting on the intake manifold, in the PTO position (but I could be mistaken). Either way, You can remove the line, and simply cap that fitting. This would definitely cause a lean condition as it would allow more air to be drawn into the crankcase in the PTO position........

  8. #8
    SPEED KILLS, BUT YOU GET THERE QUICKER Keddano's Avatar
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    Bingo, may have a winner, But also even after you fix that, do a pressure test on the engine. A bad crank seal on the PTO side could also be the problem. should be one of the two.

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