56 RDJ 18C Compression

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  • September 13, 2015 at 3:53 pm #2521
    wiscoboater
    Participant

      Hi guys
      I have a 56 Javelin RDJ 18C. The compression numbers i’m getting are 146 on the bottom and 154 on the top. Is that high of a number normal for a big twin? Seems kind of high, but all I did was hit the cylinders with some fogging oil before testing.

      Thanks, Wannabe

      September 13, 2015 at 4:47 pm #23755
      wedgie
      Participant

        Yeah, adding fogging oil will give a high reading. Run the motor for a while, let it cool down and re test.

        September 13, 2015 at 4:51 pm #23756
        chris-p
        Participant

          Yeah Ive never seen one that high!

          I like to test dry. The only way to give you a base line of the health of your motor. If you add fogging oil, it cheats the results, and, there is no way in heck to mimick exactly how much you added, or what you did, in the future, if you suspect a problem and re test. You don’t have a baseline.

          Test them dry. A light mist of something like wd40 should not cheat it too much, if you are worried about a motor that has been sitting for many years.

          September 13, 2015 at 5:04 pm #23758
          wiscoboater
          Participant

            Does it make a difference if I test with the pull or Estart?

            September 13, 2015 at 5:26 pm #23766
            chris-p
            Participant

              Should be tested with what is available on the motor, so electric start motors tested with electric starter, manual start motors tested with manual starter.

              September 13, 2015 at 9:18 pm #23783
              opposedtwin
              Participant

                US Member

                I’ve had 15 or so rd/big twins and have never seen higher than 125. 105 or higher is best, o/w you may have issues with poor low idle, etc

                September 13, 2015 at 9:37 pm #23785
                chris-p
                Participant
                  quote opposedtwin:

                  I’ve had 15 or so rd/big twins and have never seen higher than 125. 105 or higher is best, o/w you may have issues with poor low idle, etc

                  I find the same, average motors about 105 or so. The real nice ones with low hours, 125….maybe 130 ish?

                  September 14, 2015 at 10:55 am #23835
                  wiscoboater
                  Participant

                    I have to replace all the fuel and air lines and clean carb before I can try a test fire. I’m just glad after sitting for 24 years or so it turns as well as it does. And the compression is not 0.

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