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  • compression 96 GTX

    i put a new top end in my 1996 GTX 787 I have only been able to run it for about an hour. I took a compression test the other day and I got 150 on the mag side and 143-144 on the pto side. is it lower on one side because the rings might not be set yet? The carbs are rebuilt and i am using premix i took the oil injection off. is there something else that could be causing tthe low compression or is it fine the way it is?

  • #2
    Re: compression 96 GTX

    A +/- 10% tolerance is acceptable, your compression is fine..
    John Kubiak
    Powersports Technical Training Professional
    Las Vegas, Nevada
    Sea-Doo Tech 13736
    PWC Tech since 1988 (22 years)
    PowerSports since 1976 (34 years)
    NEVER BUY TIRES AT A "BLOW-OUT" SALE
    Please do not use Private Messaging, use the forums.

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    • #3
      Re: compression 96 GTX

      Thank you John I just wanted to make sure eveything is in order i don't want to have to rebuild this motor again. There was one problem that i was having when i took it out. The ski ran great up to about 3/4 throtle after i was riding around for a while i went WOT for about 10 sec. then the ski died the 12V low was flashing. I had the ski towed in and I brought it home and went through the carbs again the pop off was to high so i reajusted it and got it to around 35 psi in both carbs. I also put a new rectifier in because i wasnt sure if the ski died because the battery wasnt chrging or not. Do you think I should be go to go now or should i look into something else before i take it out again?

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      • #4
        Re: compression 96 GTX

        I do not agree. 10% is alot.
        I consider 5% about the maximum allowable difference between clyinders.

        I am not saying it will not run, but with 10%, you have some damage to that piston.

        It sounds to me like you got a mild siezure from not getting enough fuel to that clyinder for whatever reason..
        Bill O'Neal <br>
        WCM
        <a href="http://www.watercraftmagic.com"

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        • #5
          Re: compression 96 GTX

          Industry Standard Is 10%, RACING STANDARD is 5% or less. In this case he is at 5% now, so again there should be no issue.
          John Kubiak
          Powersports Technical Training Professional
          Las Vegas, Nevada
          Sea-Doo Tech 13736
          PWC Tech since 1988 (22 years)
          PowerSports since 1976 (34 years)
          NEVER BUY TIRES AT A "BLOW-OUT" SALE
          Please do not use Private Messaging, use the forums.

          Comment


          • #6
            Re: compression 96 GTX

            Bill could the wrong pop off pressure in the carbs cause it to not get the right amout off fuel at WOT? because i have been through everything on the carbs and i checked all the fuel lines everything else seems good. the pop off pressure was high so i got it down to around 35 psi in both carbs now.

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            • #7
              Re: compression 96 GTX

              Pop-off pressure is an adjustment to compensate for a momentary extreme lean hesitation condition as the throttle is moved just off of idle. So, the pop-off will not affect fuel flow as you describe.
              John Kubiak
              Powersports Technical Training Professional
              Las Vegas, Nevada
              Sea-Doo Tech 13736
              PWC Tech since 1988 (22 years)
              PowerSports since 1976 (34 years)
              NEVER BUY TIRES AT A "BLOW-OUT" SALE
              Please do not use Private Messaging, use the forums.

              Comment


              • #8
                Re: compression 96 GTX

                No not really. Once the needles are wide open, they flow the full amount of fuel. If the pop off were too high to open the needles, you would never see full speed.

                Pop Off pressure effects the transition from low to high circuits and when too high, will cause a pause in the throttle response when getting into and out of the throttle. That is where too high or too low PO pressures will show up most.
                Bill O'Neal <br>
                WCM
                <a href="http://www.watercraftmagic.com"

                Comment


                • #9
                  Re: compression 96 GTX

                  I am just trying to find out why it wouldnt be getting enough fuel to the pto carb that is the piston that messed up last time so that is why i went to premix. when the pto piston messed up the first time i found that one of the bolts on the RV cover was really loose could that have caused the piston melting? If that was the reason the first time what could i check this time so it doesnt happen again?

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                  • #10
                    Re: compression 96 GTX

                    Well, even though you found a bolt loose, you should never assume that was the ONLY problem that you may have had.

                    I'd be going back through that rear carb again if it were in here. If I found nothing, I would be looking for a crankcase air leak somewhere.
                    Bill O'Neal <br>
                    WCM
                    <a href="http://www.watercraftmagic.com"

                    Comment


                    • #11
                      Re: compression 96 GTX

                      Let's not assume the obvious first of all. What exactly are you calling a top-end job? Did you mearly throw new rings in, or were the cylinders bored/honed? If they have slight variances in the clearance, or ring gap, or head cc's - that will effect the absolute numbers as well.
                      SBT Tech Support is here to help with your problems.
                      We try to answer each question quickly and accurately.
                      Please do not use Private Messaging for Tech Support, use the forums.

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                      • #12
                        Re: compression 96 GTX

                        Well the motor was a complete rebuild form another company who I will never do business with again if my life depended on it. I whish i knew about SBT last year. So anyway the pto piston went last year then i got it back and then the mag piston went last year also then this year the pto piston went again so I found out that it was the oil pump i hope so i went to premix. In the mean time i rebuilt the carbs put all new fuel filters in, cleaned raves, cleaned mag, cut back spark plug wires, new rotory valve, new rectifier, new starter, new fuel pick up, and then i even went through the carbs again and all the fuel lines I just dont want to have any more problems. So back to the rebuild I gave the machine shop both pistons (new) and he matched them to the cylinders so there are new pistons new rings and bored cylinders 1mm over and new ristpin bearings. If there is any thing else that i could check before I take it out again please let me know. Thanks again everyone.

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                        • #13
                          Re: compression 96 GTX

                          You should check the clearance between the piston and the rave valve since you bored 1mm.... The rave valve sits "deaper" in the jug now

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                          • #14
                            Re: compression 96 GTX

                            Sorry i forgot to say that i had the raves milled down so nothing would hit.

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                            • #15
                              Re: compression 96 GTX

                              I think given the variables of your project, you are probably fine, and there are just a lot of contributing factors to a slightly different number. I wouldn't assume damage, personally.
                              SBT Tech Support is here to help with your problems.
                              We try to answer each question quickly and accurately.
                              Please do not use Private Messaging for Tech Support, use the forums.

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