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Thread: JD compression rates

  1. #1

    Default JD compression rates

    I know there are many variables and I don't know what internals are in my JD but can anyone tell me the normal compression rates for the JD? Mine started running bad and when checked I got 25 psi out of the rear cylinder and only 15 psi out of the front cylinder.

  2. #2
    Join Date
    Apr 2003
    Seaford, NY


    Sounds very low. You should have at least 75 psi. Confirm your guage accuracy.

  3. #3
    Join Date
    Jul 2009
    Sylacauga, AL. USA.


    My racer, runs about 55 to 60 lbs warm, with an original iron piston/rings and tapered bore, since I built it about 5 year ago. Try lapping the valves, and the intake housing.
    Last edited by dlm32; 08-19-2016 at 07:42 AM.
    David Morrill
    Sylacauga, AL. USA.
    AMCA #15284

    Deadly Dave's Blog
    Sharing the Lost Stories of Early American Motorcycling.

    1921 Harley-Davidson Model J Racer

    It will break a hundred, if you drop it from a plane!

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