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  1. #1

    It started with fuel lines

    I just started to replace the gray fuel lines. Now I'm rebuilding the carbs. So I was thinking if I have the carbs why not check the compression on the cylinders 118 -120. I guess the rings are bad, somewhere somebody posted that they should be 160 psi. If I have to replace the rings how do I line up the gaps. I'm also noticing that the gasket between the cylinder and the block there are little hold that means something. Each hold is the thickness of the gasket, if I reduce the thickness would I increase compression like from .024 to .012 inch.


  2. #2
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    use the same gasket

    the holes indicate the gasket thickness, unless you are milling the head or doing other hi performance work you should use the same gasket. This controls the combustion chamber volume, which is really tricky to measure. Don't worry about lining up the rings, there are little locating pins in the ring grooves that control that.

    guessing at this stage means you could be guessing again when things go wrong., but since the jugs are already off....

    post some clear pics of everything and rely on the expertise of the experts here to push you in the right direction.

  3. #3
    RumRunnerRon's Avatar
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    Before just slapping a set of rings in that thing and calling it good. You should get your self a bore gauge and caliper then check the cylinders for out of round and taper. That's just one of a few things that come to mind.

    +1 on the pics. These guys on here can tell you a lot just from pics. If you are going to all the trouble to tear it apart ya might as well do it right the first time.

  4. #4
    flyin' the friendly skies airbornexp's Avatar
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    make sure when testing compression that you have a strong battery. If you have a weak battery it will show a false positive on your compression. plus if you dont have a good compression gauge it will also show a difference but the difference between the two cylinders seems ok. how many hours are on the engine?

  5. #5
    130 hours. The crank and bottom end looks good no brown color or crap. I'll take pics today. I'll post them tomorrow.

  6. #6
    steach's Avatar
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    Quote Originally Posted by wolfman1998 View Post
    130 hours. The crank and bottom end looks good no brown color or crap. I'll take pics today. I'll post them tomorrow.
    I would not take it apart unless you have to. As Airborne said, I expect the battery is low or gauge is low.

    A good way to examine the top end is to pull the RAVEs and look into the holes with a flashlight at the piston/rings. By pulling the plugs you can rotate the PTO while looking.

  7. #7
    The battery is strong. I trickle charge it when it's not in use. I took it out 3 weeks ago and it was bogging down, that is why I started with the fuel lines (gray) and the carbs. The filter/ jet was plugged also. The compression tester is new. I had the shop check the cylinder and their right on the money, he wanted the pistons but I didn't pull the ring pins.

  8. #8
    flyin' the friendly skies airbornexp's Avatar
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    could be leaky needle and seats too...pull them and get new ones or at least replace the o-rings

  9. #9
    I got a complete rebuild kit Mikuni MK-BN38/44 SPR

  10. #10
    The compression should be 150 at sea level. If you are at a higher elevation, it can be less. My 951 reads [email protected] 4500 ft, all others read 120-125.

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