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  1. #1
    silvercrxsir's Avatar
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    rather odd issue with compression

    Had my 01 gp800r out yesterday ran fine all day,but all of a sudden was weak and wouldn't rev past 6k,so I loaded it up and went home,pulled the plugs and noticed rear cyl is running rich,did a quick compression test and got 140 rear 120 front,i tested 3 times to verify this and same results every time,motor is stock and I tested compression atleast 3 times this year and both cylinders always sat at 120, what would cause this increase,maybe a rich condition or excess fuel,maybe because the plug fouled?the plug didn't look real bad but was fairly wet and swapping them made it run a lot smoother on the trailer,any clue to the compression jump


  2. #2
    Yamaha artisan Cutlass's Avatar
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    Maybe there is excessive oil in that cylinder adding to the rings ability to seal and giving you a higher reading. ??

  3. #3
    mudslanger's Avatar
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    Check your power valve make sure it isn't broke.

  4. #4
    Happily Self-Employed WFO's Avatar
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    Quote Originally Posted by mudslanger View Post
    Check your power valve make sure it isn't broke.
    +1

  5. #5
    silvercrxsir's Avatar
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    I was almost starting to think the same thing,I was planning on pulling them out tomorrow to inspect and clean

  6. #6
    silvercrxsir's Avatar
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    That was it,I pulled the exhaust and the valve was broke,now my question is where to go from here,from what I could feel,it feels like the piston is fine and can't feel any areas of contact or scratches,what causes them to break,do they just crack from stress,I'm gonna rig up a mirror for a better look,if everything looks ok should I just drop another in and put her back together

  7. #7
    silvercrxsir's Avatar
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    I got the valve out,it broke where the stem meets the paddle part,the valve was clean at the very tip but the machining marks were still in it,no nicks or gouges at all on the valve it looks like a clean break.I will try to get some pics up,I managed to remove it without pulling the head,so im pretty sure at this point I will toss another valve in and reassemble and clean the other one,any clue what causes this,it looks like stress or fatigue,no sign of impact

  8. #8
    Happily Self-Employed WFO's Avatar
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    I have seen them rub on the side of the piston, and also seen them catch the top of the piston on compression stroke, causing the rings to be pinched by the top of the piston

  9. #9
    silvercrxsir's Avatar
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    I just pulled the front one out to the same result except the petals is long gone and appears to have been missing for a while which would explain the weak hole shot,I never experienced any problems with this ski or never heard any weird noises ever and haven't pulled the head,but from what I can see with a mirror everything looks ok and comp is at 120 for both,piston skirts look a little shiny but no scoring,what do u recommend

  10. #10
    mudslanger's Avatar
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    I wouldn't take a chance of messing up the motor. I would pull at least the 2 cylinders and inspect the cylinders and pistons. Too much money to miss something and rings aren't too expensive.

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