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  1. #1
    Pain is fear leaving your body.... rlovebk's Avatar
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    Compression check Maybe ut oh??

    I just did comp. ck. on my 94 750 slt Came up 80 lbs on each cylinder. Is this low?? Seemed like it based on what I've been reading around here. Thanks folks. You all are a big help.


  2. #2
    Moderator beerdart's Avatar
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    80 is low but they are all with-in 10% so that is good. Try a different gauge some gauges read lower than others.

  3. #3
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by rlovebk View Post
    I just did comp. ck. on my 94 750 slt
    Came up 80 lbs on each cylinder. Is this low??
    Seemed like it based on what I've been reading around here...
    Are all three cylinders showing the same compression numbers?

    You sure your gauge is accurate?

    You tested with a strong battery, and held the throttle wide open while cranking until the reading stabilized?

    Good compression numbers, using a known accurate gauge and strong battery, would be 120PSI or higher, and should be consistent across all cylinders, within 5-10 percent.

  4. #4
    Pain is fear leaving your body.... rlovebk's Avatar
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    Brand new gauge. Carbs are off. Putting back on tomorrow.

  5. #5
    Pain is fear leaving your body.... rlovebk's Avatar
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    all three read btwn 75-80 lbs. Maybe gauge is funky.

  6. #6
    Does your guage screw into the head the same distance as the spark plugs?? ( some guages have short reach threads on them)

    If not then thats why your readings are low !!!!!!

  7. #7
    Race, wreck, repeat delagem's Avatar
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    My experience has been that motors become difficult to start under 100 psi compression, and won't run below 85-90 without help from starting fluid/ether.

    Is your ski running? If so, you've probably got a bad gauge or a leak on the hose somewhere.

    Missing or cracked O ring on the end that screws into the head? Loose hose where it threads into the gauge?

    Michael

    EDIT: Oh, wait a minute, carbs are off? Are reeds off too? Unsure if that would cause low compression, but I sure think so!

    BTW, I have tested compression with hot vs cold motor, open vs closed throttle, etc. I never got more than a few pounds difference by doing any of those... But I never tried it without reeds!
    Last edited by delagem; 09-06-2008 at 03:32 PM. Reason: Had another brainstorm. Brilliant!

  8. #8
    Quote Originally Posted by rlovebk View Post
    Brand new gauge. Carbs are off. Putting back on tomorrow.
    Are your reeds still in?

  9. #9
    Lake Mead Bum & BTLS Mark starflight's Avatar
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    While I was building mine I tested w/o reeds installed & found 150 psi on all 3. Measured the same when the reeds, intake and carbs were installed.

  10. #10
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    True reading should be taken on a warm engine. Cold engine will yield lower compression due to every man's enemy..... shrinkage!!!!!

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