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  1. #1
    CB750 Member Mars's Avatar
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    Oct 2017
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    Rochester, NY
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    12

    Unhappy Low compression on cyl. 4 only - Bike sat for about 20 years

    Hello all,

    I'm in the process of getting a 1979 CB750L back up and running after something it sat for about 20 years in a shed.

    I've got it running pretty well actually but when syncing the carbs notices #4 cylinder a little wonky. Have driven it just a couple miles.

    Dry compression test showed very even 149-150 psi on cylinders 1-3 and 30 psi on cylinder #4.

    Wet compression test on cylinder 4 showed 195 psi. I'm surprised its even possible to get that high, but that's what it read.

    I'm assuming that it will need new piston rings, but one person suggested leaving Marvel Mystery oil in the chamber over the weekend and see if it loosens up the rings and drive it around a bit.

    Am I being overly optimistic I can avoid a rebuild? Or will I risk doing more damage?

    Thanks

  2. #2
    CB750 Member amaskedman's Avatar
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    May 2018
    Location
    Washinton
    Posts
    21
    thats not terribly low. Mine is in the 120s.

  3. #3
    CB750 Member Mars's Avatar
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    Oct 2017
    Location
    Rochester, NY
    Posts
    12
    Quote Originally Posted by amaskedman View Post
    thats not terribly low. Mine is in the 120s.
    Cylinders 1-3 look fine, it is #4 that worries me at 30 psi when tested dry..

    and ~200 when wet, but I may have had too much oil in the cylinder to get a proper reading.

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