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Need help bad!!!!! with piston compression

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  • Need help bad!!!!! with piston compression

    Well I finely got my super jet on friday night. it's a 98. the guy that I bought it from lives about 9 hour away and delivered it for me. When the boat was droped off I didn't have a compression gauge to check the compression.so I didn't know about the problem till Saturday. I still haven't given the guy money yet so I could give him the ski back if it is no good. My problems is that when I did a compression test on the motor I got some bad numbers. pto= 165PSI, Mag =135 PSI. the boat has a ADA head on it and after pulling the head I found out that it has 33cc domes which should produce 200 psi of compression from ada website. I looked at the cylinder wall of the mag pistions and there are some very very fine marks, I can feel them when I run my finger on the wall. Would this mark make me lose 30 lbs of compression? My other thought for the compression being lower for both pistons, under 200 llbs, was that the cylinders were bored. This would cause the compression to drop right? Is it possible that the compression was lower on the one cylinder because of a bad o-ring? any sugestions on were one could find o-rings? Are they somewhat standard O-rings? or is it best to get a set from ada?

    I hope that I didn't take buttkicking on buying this super jet.

    Thanks for your guys help
    Nathan

    [ April 07, 2002, 11:46 PM: Message edited by: cdooguy ]

  • #2
    Sorry to say but that engine is most likely du for at least a top-end job. Yes a blown head-o-ring can cause a bad compression reading, but it's easy to see if it or the surfaces are damaged. Since you see cylinder scoring you know the rings and piston have been damaged and MOST likely your low compression is due to seizure damage. Best thing to do right now, is tear it apart for repairs if you are going to keep it.

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    • #3
      Well, in this case, with his aftermarket heads, I assume he's asking if compression will riase or drop with the subsequent instllation of larger pistons. And unless he ALSO had the head recut, it will raise.

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      • #4
        thanks Brian!

        can you clear this question for me?
        if you bore a cylinder will the compresion drop?

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        • #5
          Installing larger pistons actually makes it go up.

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          • #6
            also if the ski hasnt been run in a long time then the rings could be sticking giving you a lower compression
            Rob

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            • #7
              2000 super jet compression

              What should a stock 2000 superjet produce as far as compression?

              Brand new?

              5 years old?


              Thanks:emoticonc
              :emoticbar

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