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    #1

    Confused about compression after research

    I'll start with the problem I'm having which is very low compression in 3 out of 4 cylinders after doing a top end rebuild. Yes, I did the compression test with the throttle open. Cylinders 1, 3, and, 4 had very low readings. Maybe 10 to 15 psi but my gauge doesn't start till 20. Cylinder 2 had a nice reading of 130ish though. I did a dry and wet test on the bad cylinders with no noticeable change in the reading. So that's my problem, and here's a link to my rebuild thread for more info http://www.thegsresources.com/_forum...ts-of-problems.

    Now for how I got confused. It happened once I started reading other threads on here about other people with low compression after top end rebuilds. It was very common to see responses that basically said 'don't worry about compression on a rebuilt engine till it's got some miles on it'. Now most of those types of threads either didn't give their compression numbers or they were in the 50-70 range which is significantly different from my almost 0 reading but it still confused me as I though these engines needed to be around 100 psi to run.
    Tags: compession
    #2
    Check your vales are closing

    Comment

      #3
      Originally posted by RGM View Post
      Check your vales are closing
      I'm just going to pull the top apart again. If 3 cylinders have valves that won't close/seat properly then I can't think of a way to fix that with the head still on the engine.

      Comment

        #4
        Originally posted by MechMessiah View Post
        I'm just going to pull the top apart again. If 3 cylinders have valves that won't close/seat properly then I can't think of a way to fix that with the head still on the engine.
        Well.adjusting your valve clearances comes to mind
        You say you did a top end rebuild, did you lap the valves?
        1978 GS 1000 (since new)
        1979 GS 1000 (The Fridge, superbike replica project)
        1978 GS 1000 (parts)
        1981 GS 850 (anyone want a project?)
        1981 GPZ 550 (backroad screamer)
        1970 450 Mk IIID (THUMP!)
        2007 DRz 400S
        1999 ATK 490ES
        1994 DR 350SES

        Comment

          #5
          Originally posted by Big T View Post
          Well.adjusting your valve clearances comes to mind
          You say you did a top end rebuild, did you lap the valves?
          I should have finished that thought. I can't think of a way to fix that with the head still on that I haven't done yet. The Valve Clearances were adjusted and triple checked and yes also lapped. The link to my rebuild thread is in the first paragraph of this thread and also here.

          Comment

            #6
            Does the engine fire? How does it sound and ride?
            Richard
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            GS1150 EF bought Jun 2015
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            Previously owned: Suzuki GS750 EF (Canada), Suzuki GS750 (UK)(Avatar circa 1977), Yamaha XT500, Suzuki T500, Honda XL125, Garelli 50
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            Comment

              #7
              Did you check the valves for leakage after you lapped them in? Have you put new piston rings in? If you did, did you hone the bores! The compression readings are poor on 3 cylinders. Are the valves clearances set correct?
              Did you check the cylinder head for warpage?
              The big guy up there rides a Suzuki (this I know)
              1981 gs850gx

              1999 RF900
              past bikes. RF900
              TL1000s
              Hayabusa
              gsx 750f x2
              197cc Francis Barnett
              various British nails

              Comment

                #8
                Going right outside the box here.
                Might be worth having a close look at how the valves return before taking off the head. Is the return smooth all the way back up and can the buckets be moved easily when the cam comes off them at the top of the return. #2 is your comparison.
                Carbs, was anything done to them. If you are going in I would try another compression test with the carbs off before removing the head.
                Is the exhaust free and clear ?
                97 R1100R
                Previous
                80 GS850G, 79 Z400B, 85 R100RT, 80 Z650D, 76 CB200

                Comment

                  #9
                  Originally posted by londonboards View Post
                  Does the engine fire? How does it sound and ride?

                  Thanks for the suggestions everyone but that wasn't why I made this post. I'm confused about comments like the one above and trying to get some clarification. I was always taught that a combustion engine needs three things to run. Spark, Fuel, and Compression. With out one the engine just won't run.

                  However when I was doing research about very bad compression I kept seeing people ask "Will it run/start?". Were these bad questions, people not paying attention to details of the situation? Or have I been taught wrong about compression? Can you run an engine with drastically bad compression?
                  Last edited by Guest; 08-09-2016, 11:45 AM.

                  Comment

                    #10
                    If anyone is interested I found the problem. It's been posted in my rebuild thread here. I'm still curious about this compression quagmire though. Isn't it a fundamental requirement for a combustion engine?

                    Comment

                      #11
                      404 Not Found
                      http://vmaxoutlaw.com/tech/leakdown_tester.htm


                      A leak down tester would of found that out right away.

                      Comment

                        #12
                        Originally posted by RGM View Post
                        http://vmaxoutlaw.com/tech/leakdown_tester.htm

                        A leak down tester would of found that out right away.
                        I found it right away when I pulled the head, which needs to be done to replace the gasket anyway.

                        Comment

                          #13
                          So what is the root cause of the problem? Undertorqued head nuts or a bad gasket?
                          Ed

                          To measure is to know.

                          Mikuni O-ring Kits For Sale...https://www.thegsresources.com/_foru...ts#post1703182

                          Top Newbie Mistakes thread...http://www.thegsresources.com/_forum...d.php?t=171846

                          Carb rebuild tutorial...https://gsarchive.bwringer.com/mtsac...d_Tutorial.pdf

                          KZ750E Rebuild Thread...http://www.thegsresources.com/_forum...0-Resurrection

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