Do these compression readings make sense?

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  • shadow
    • Feb 2026

    #1

    Do these compression readings make sense?

    I've got a rough '78 1000 that I'm still trying to figure out what I'm going to do with. I'm in no hurry to get it on the road, and I don't want it to turn into a complete money pit, so it will be a slow project. It runs, but it's got a busted exhaust bolt, broken off flush, and I can't get to it because it's behind the frame. Eventually, if I continue with it, I want to pull the engine for cleaning and paint, but for now, I was thinking of just pulling the head to try and get at the busted bolt.
    I thought I'd check compression, before I get too far into this.

    These were my readings on a warm engine:
    cyl #1-150
    cyl #2-140
    cyl #3-125
    cyl #4-130

    I was concerned about the compression being too low, but if anything, these values seem a little high to me, Especially #1 and #2.
    I was hoping some of you more experienced guys could chime in and give me your opinions.
  • SqDancerLynn1

    #2
    Those readings are not too bad, I would not be concerned until the motor has run for at least one hour and the valves have been adjusted

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    • rustybronco
      Forum LongTimer
      Bard Award Winner
      GSResource Superstar
      Past Site Supporter
      • Jul 2005
      • 14961
      • Marysville, Michigan

      #3
      17% difference between lowest and highest cylinder. still above the 100 psi service limit. standard compression is from 128 to 184 Psi with no more than 28.5 psi lowest to highest cylinder. among other things, could be carbon build up that's the cause of the differing readings.

      check the valve clearances and fix the broken stud.
      De-stinking Penelope http://thegsresources.com/_forum/sho...d.php?t=179245

      http://www.thegsresources.com/_forum...35#post1625535

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      • shadow

        #4
        Thanks for the quick responses. So I shouldn't worry about it. I'll try to fix the broken bolt, check valve clearances, clean carbs etc. I know I have to fix the exhaust leak, before I'll ever get it to run right.

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        • Guest

          #5
          Those numbers scream one thing!
          RUN ME!

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          • shadow

            #6
            Originally posted by chef1366
            Those numbers scream one thing!
            RUN ME!
            I'd love to, but I'm afraid it'll be awhile until this bike hits the road. Even if it was running, it's snowing as I type this.
            It still needs alot of work, but it's encouraging that it probably doesn't need a rebuild.

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            • rustybronco
              Forum LongTimer
              Bard Award Winner
              GSResource Superstar
              Past Site Supporter
              • Jul 2005
              • 14961
              • Marysville, Michigan

              #7
              One of these days I'll learn to say things as succinctly as Bill did.
              De-stinking Penelope http://thegsresources.com/_forum/sho...d.php?t=179245

              http://www.thegsresources.com/_forum...35#post1625535

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              • SqDancerLynn1

                #8
                If you do pull the head ? Pull the cylinder too. Don't necessarily need to change the rings? How many miles ?? The base gasket should be replaced, removing the head will disturb it and cause it to leak.

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                • Guest

                  #9
                  Originally posted by rustybronco
                  One of these days I'll learn to say things as succinctly as Bill did.
                  Right Dale

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                  • shadow

                    #10
                    Originally posted by SqDancerLynn1
                    If you do pull the head ? Pull the cylinder too. Don't necessarily need to change the rings? How many miles ?? The base gasket should be replaced, removing the head will disturb it and cause it to leak.
                    Thanks for the tip. If I do end up pulling the head, I'm sure I'll have a bunch more questions.

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                    • 850 Combat
                      Forum Guru
                      Past Site Supporter
                      • Sep 2006
                      • 6018
                      • Spokane, WA, and Hampden, ME

                      #11
                      Originally posted by SqDancerLynn1
                      If you do pull the head ? Pull the cylinder too. Don't necessarily need to change the rings? How many miles ?? The base gasket should be replaced, removing the head will disturb it and cause it to leak.
                      Might not. I did a head gasket on my 1000G and didn't pull the cylinder. So far so good.

                      Those compression readings don't look like needing rings to me, of course if the cylinder was pulled, a honing and new rings would be in order. I agree that it would be a good practice to pull the block.
                      sigpic Too old, too many bikes, too many cars, too many things

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