720 compression loss
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  1. #1

    720 compression loss

    I don't get it, this thing was running awesome for 20 minutes and then it died in the water. Told it back and did a compression test 150 psi on the PTO 60 psi on the mag.

    Last week it had 150 PSI in both cylinders so I assumed when I took the top end apart I would find the piston broken up. But it looks to be in perfect shape..

    What the hell would cause the mag cylinder to lose compression?? The cylinder walls look fine The rings look fine. No signs of detonation.

  2. #2
    PWCToday Regular Old man rookie's Avatar
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    Re: 720 compression loss

    Head gasket failure?

  3. #3

    Re: 720 compression loss

    Quote Originally Posted by Old man rookie View Post
    Head gasket failure?
    I put it back together with all new gaskets a few minutes ago. Still 150 psi on the pto 60 psi on the mag. I don't get it. The pistons, cylinder walls, etc look perfect. Oily, but perfect.

    I guess it must be the rings but.. If it was the rotary valve I guess it would be low in both. It's a single carb, so it's unlikely to be anything related to the intake side.

    Don't get how it could have ring failure after 20 minutes on the MAG side, without any signs of trauma in the cylinder.

    mag_2.jpgmag_1.jpg

  4. #4
    PWCToday Newbie
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    Re: 720 compression loss

    I am pretty sure it has a rotary valve for each cylinder??

  5. #5
    PWCToday.com Is My Home Away From Home Bionic racing's Avatar
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    Re: 720 compression loss

    Stuck a ring , did you redo the top end before failure ?

  6. #6
    PWCToday Regular
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    Re: 720 compression loss

    I would look for a stuck ring

  7. #7
    PWCToday.com Is My Home Away From Home Grumpy Old Guy's Avatar
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    Re: 720 compression loss

    Those pistons look old...

  8. #8

    Re: 720 compression loss

    Quote Originally Posted by Chuck_b24 View Post
    I am pretty sure it has a rotary valve for each cylinder??
    Nah single rotary valve on a 720.

  9. #9

    Re: 720 compression loss

    Quote Originally Posted by Grumpy Old Guy View Post
    Those pistons look old...
    720 from 2001. I pulled it out of a GTS that had 50 hours on it. Had 150 psi in both cyls. This is the one that I sent you the carb to redo.

    It would start immediately and die every couple minutes. I replaced all the fuel hoses, and the pulse line. Rebuilt the jet pump (wear ring was pooched). Boat ran amazing, came up on plane, ran from one side of the lake to the other and just died.

    Plugs look black and sooty, no signs of detonation. Just don't understand why a ring would fail when everything else seems ok.

    Ordered a top end kit. I'll check the cylinders with bore gauge when I tear it down, but visually the cylinder looked perfectly fine. Oil residue on the top of the chamber, so I don't think it was an oiling issue.

    I was thinking maybe an overly rich fuel condition might have washed the cylinder wall and wiped out the rings, but being a single carb why wouldn't it do that to BOTH cylinders?

    -- Joe

  10. #10
    PWCToday Regular
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    Re: 720 compression loss

    the rings float in the grove in the piston, if one sticks in you loose psi

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