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  1. #1

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    Question Please help both cylinders low compression

    Hi I am a new pwc owner. I just purchased a 1996 seadoo hx and I know they should have 150psi for compression doing my research but the one I have just purchased has 90psi in both cylinders wich I find to be odd because usually if a ring goes bad there would be a difference between the two cylinders right? The previous owner had told me he had just put new rings in it the beginning of last season so I was wondering if anybody could give me an idea of what's going on and why I should check. Thanks
    Last edited by Nrbball4391; 03-04-2013 at 07:53 PM. Reason: Spelling


  2. #2
    Wondering if they honed the cylinder or just stuck rings in there

  3. #3

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    Quote Originally Posted by yamahayz250rider View Post
    Wondering if they honed the cylinder or just stuck rings in there
    That's what I'm thinking he did but even still shouldn't there be a difference in psi between the two cylinders? I'm thinking maybe he reused all the old gaskets and there's a leak?

  4. #4
    chartersj's Avatar
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    sounds like at least the top end is going to have to come off to see whats going on inside, could speculate all day. Sounds like the guy did the work himself (which isnt always a bad thing) but i personally have a hard time trusting backyard mechanics work

  5. #5
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    if you used a cheapo compression gauge or didn't have the thottles wide open..or there is a rat nest in the airbox....

    don't expect a 15 year old engine to be able to pump up to 150 psi..new rings won't fix an out of round cylinder bore.

    Oh yeah..check the front or back of the block for an SBT sticker..could be that 100-120 is the best you are going to see.

    of course the old trick of putting a little injector oil into each cylinder and remeasuring might point to your problem.

  6. #6
    rhyne717xp's Avatar
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    Which kind of cylinder base gasket was used? That can affect compression, a weak battery/starter that cant crank the engine hard enough could affect your readings. My 95xp with the 720 engine only yeilds 135 psi cold engine compression. Once heated up i get 140ish psi. Engine runs great, the compression stays the same season after season. I may be wrong but a factory 717 engine is a little lower than 150 unlike its big brother 787.

  7. #7
    What's your altitude?

  8. #8

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    It was the first start of the season it's been sitting unstarted for 6 months maybe that's why the compression was so low? The motor was cold. I'm going to run it for a while and warm it up and see if there's a change in compression and if not ill pull the top end apart and rebuild it. The motor runs and sounds great with great throttle response I'm just hoping that's not going to change once I drop it in the water. Thanks everyone for all your help I really appreciate it

  9. #9
    wkuadam07's Avatar
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    I saw a motor have equal but low compression a while back. The rotary valve timing was wrong. It ran great and picked up great on the hose and sounded normal. Wouldn't start in the water and had low compression. I timed the rotary valve correctly and it went from 93/95 compression to 135/139 compression. It started in the water and ran great.

  10. #10
    Myself's Avatar
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    Oh boy! You're gonna love that HX once you learn how to ride it. Do yourself a favor and pull the engine for a complete teardown. Super easy and you can can inspect the whole motor. You never know what you have unless you do. Mark the mounts and location of shims before pulling. Typically I find the 720's have loose or worn rear crank bearings. You can pull them yourself with a puller and replace them with new ones for about $25. You will also be needing new crank end seals as yours are now 17 years old. The 720 comes oem with almost non-existent port chamfers and as they get some hours they become knife edged and can grab a ring. You can also do a little case porting and re-shape the boost port roof while it's all apart if you're looking for free extra power. Holler if you need help, I know my around the smaller Rotax powerplants.

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