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  1. #1

    Can excessive exhaust gas back pressure cause detonation/engine ping?

    I just had my 97 SLT 780 (Blue Fuji) rebuilt. (by SBT…leak down test preformed prior to install)

    Good compression .5 over sized pistons. 144 psi Mag, 150psi Center, 130 psi PTO

    New rebuilt kits in all carbs including new needle and seat and set to spec.

    After experiencing a midrange “ping” I scoped the PTO cylinder and noticed an ever so slight scoring on the cylinder walls..(luckily with no large amount compression loss)

    Adjusted the carbs to make sure there is no lean settings.

    However, I discovered my exhaust crossover from the water box has eroded in the interior and has collapsed internally.

    I’ve read on line that any restriction or blockage in the exhaust system will increase back pressure, which means that the hot exhaust gasses are kept in the combustion chamber for longer, thus increasing chamber temperature and increasing the likelihood of detonation.

    Seems logical…but has anyone had firsthand experience with this being the case? Or Am I looking at more of a timing issue?. This machine hasn’t been on the water long enough to even remotely consider the crank being out of index.

    Any help is greatly appreciated.
    Thanks
    Bobby


  2. #2
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    Quote Originally Posted by polaris780slt View Post
    I just had my 97 SLT 780 (Blue Fuji) rebuilt. (by SBT…leak down test preformed prior to install)

    Good compression .5 over sized pistons. 144 psi Mag, 150psi Center, 130 psi PTO

    Bobby
    I'd be making a phone call to SBT about those compression readings a 20lb difference is NOT acceptable in a rebuilt motor.

    I'd check the readings again

  3. #3
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by polaris780slt View Post
    I just had my 97 SLT 780 (Blue Fuji) rebuilt. (by SBT…leak down test preformed prior to install)

    Good compression .5 over sized pistons. 144 psi Mag, 150psi Center, 130 psi PTO
    ...
    I thought SBT rebuilds always used 1mm oversized pistons

  4. #4
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    due to the state of the economy..they can't afford to use those bigger and much more expensive full over-sized pistons anymore

  5. #5
    Quote Originally Posted by K447 View Post
    I thought SBT rebuilds always used 1mm oversized pistons
    I should have mentioned that SBT did a great job with the rebuild (as they always do). The engine arrived out of the box all good with pre install compression check with 145 psi in all cylinders. And, yes typically SBT rebuilds always use 1mm oversized pistons.

    However, but because they also shave/machine the heads as Standard Operating Procedure for their rebuilds which results in a lower (120 psi) compression (borderline service limit). I always request .5 over with original OEM heads to get the compression back up close to normal as using the original OEM heads with 1mm over is too much over squish.


    So, having clarified that..I discovered my exhaust crossover from the water box has eroded in the interior and has collapsed internally.

    I’ve read on line that any restriction or blockage in the exhaust system will increase back pressure, which means that the hot exhaust gasses are kept in the combustion chamber for longer, thus increasing chamber temperature and increasing the likelihood of detonation.

    Seems logical…but has anyone had firsthand experience with this being the case? Or Am I looking at more of a timing issue?. This machine hasn’t been on the water long enough to even remotely consider the crank being out of index.

    Thanks for your help
    Bobby

  6. #6
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    exhaust issues similar to your, be it vermin, clogged perforated pipes ( from overly mixed fuel and crummy oil) to internal collapse in both waterbox and rubber hoses, anything that changes air flow characteristics from the expected will result in all sorts of engine problems, from unsolveable flat spots, hard hot starts to overheating.

    If I'm not mistaken SBT cranks get welded, so a safe bet it's the restricted waterbox causing your problems

  7. #7
    Quote Originally Posted by nmpeter View Post
    exhaust issues similar to your, be it vermin, clogged perforated pipes ( from overly mixed fuel and crummy oil) to internal collapse in both waterbox and rubber hoses, anything that changes air flow characteristics from the expected will result in all sorts of engine problems, from unsolveable flat spots, hard hot starts to overheating.

    If I'm not mistaken SBT cranks get welded, so a safe bet it's the restricted waterbox causing your problems
    Makes complete sense...I'll try new hose and waterbox...Keep you posted

    Thanks so much for your help
    Bobby

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