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  1. #1

    Low compression on allegedly refurbed engine 900zxi

    I picked up a slightly used (10 or so hours supposedly) refurbed engine for my 900zxi last summer. I finally got around to dropping it in this summer. After getting my carbs tuned and throttle cable set, I randomly decided to check the compression on the engine.

    Low and behold, the front two cylinders read about 85-90, and the rear cylinder read 30. What gives?

    The engine has the SBT serial number on it, and the head appeared as though it hadn't been removed since the refurb. I popped it off myself and the cylinders look perfectly fine. No scoring or anything.

    Would the gaskets from the intake and exhaust manifolds contribute to the compression? I only ask because, I had to remove the rear gasket between the intake manifold and crankcase, and the paper separated from the metal in the gasket. I tried to put it back together the best I could with the gasket adhesive.

    Anyh thoughts, ideas?


  2. #2
    axgrider73's Avatar
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    Are there gaskets between the reed cages and cases?

    There are several areas you can loose compression.

    One trick is to test, then put a small amount of 2 stroke oil into the plug hole and retest. If the compression raises considerably, then I would suspect the rings.

    It could also be loosing compression from a broken reed, bad head gasket, etc.

    To be honest though, 85 - 90 psi on the other 2 cylinders is fairly low as well. I believe that factory compression on those skis is supposed to be around 120 psi.

    If I were to guess, I would say that you have a seized piston due to an overheat of lack of oil. But this is just a blind guess.

    Let us know what you find out.

  3. #3
    yea there are gaskets between the reed cages and crankcase. It appears the previous owner tried to get away with just silicone for some reason. I scraped away all the silicon and installed new gaskets though.

    To be honest though, 85 - 90 psi on the other 2 cylinders is fairly low as well. I believe that factory compression on those skis is supposed to be around 120 psi.
    this was my thought exactly. I had expected much higher compression on a "refurb" motor.

    I may try and pick up some new gaskets tomorrow and see what happens.

    I forgot about the oil trick, I'll be sure to try that as well once i get the new gaskets in.

  4. #4
    axgrider73's Avatar
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    Let us know how it goes. Good luck!

  5. #5
    Well I finally got around to swapping out gaskets today and no dice. Still have low compression on the rear jug. A cap full of oil bumps it up about 5-10 psi. But it's still sitting around 30 psi. Other two are 95ish.

    Any ideas what it could be? Could it be crank related?

  6. #6
    Myself's Avatar
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    I'd say rings are burnt and stuck in the piston. You're going to have to pull the cylinder to find out.

  7. #7
    Would it just be the ring on that one piston that at ~30 psi?

  8. #8
    Could be you just need to break in the motor. I've never seen a new rebuild have that low of compression but that doesn't mean it can't happen. Also have you tried another guage some read lower than others.

    Another thing is your battery fully charged. Low turn over speed will effect readings. Just thoughts.

  9. #9
    axgrider73's Avatar
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    Time to pull the head off and take a peek.

  10. #10
    I pulled the head when I made my initial post. Cylinder walls appear to be fine.

    What should I look for that would indicate a smoked piston ring on the jug that's reading 30psi?

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