Compression readings (sad)

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  • Anonymous
    #1

    Compression readings (sad)

    Well, after posting about my clutch rattle, I read a few more threads.. and decided to do a compression/leakdown on my GS. The bike has 22,000 miles on it, and the engine was good and warm, but not hot at all. It had sat for an hour or so since the last ride.

    I was shocked to see cyl. 4 at 90 psi. It went up with oil, 110. It only got worse from there.. Cyls 1-3 sit at 70 - 80 PSI, +15-20 PSI with oil.

    I thought to myself, this can't be true! It starts right up, runs great. What's the deal? I thought it was my compression tester. So, I hook up the compression tester to the regulated 90 PSI output of my air compressor. Says 90 PSI, right on.

    I did always wonder why the bike never came close to having 'wheel lifting' power that I've heard so many GSers here talk about. (Not that I *want* my wheel off the ground!). Now I know.


    So, this winter, I'm looking at a top and bottom end (at least rings)overhaul. How much does this normally cost in parts? I'm willing to do all the labor (save machining) myself in my own garage.
    Tags: None
    • Billy Ricks
      #2
      Did you open the throttle and lift the slides?

        Comment

        • Anonymous
          #3
          No. Now I'm starting to feel dumb again.. 8O

          Will that make a significant difference? If so, I'll check again next time I have the carbs off. (soon)

          I still saw a 20 psi deficit on the leakdown, telling me that the rings have seen their day, or is that normal too?

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            • RacingJake
              Forum Sage
              • Jun 2003
              • 2698
              • Abilene Texas
              #4
              Don't forget to squirt some oil in each cylinder and check compression again. This way you'll know if it's the rings or valves.
              1166cc 1/8 ET 6.09@111.88
              1166cc on NOS, 1/8 ET 5.70@122.85
              1395cc 1/8 ET 6.0051@114.39
              1395cc on NOS, 1/8 ET 5.71@113.98 "With a broken wrist pin too"
              01 Sporty 1/8 ET 7.70@92.28, 1/4 ET 12.03@111.82

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                • Anonymous
                  #5
                  between throttle closed and throttle wide open my readings went from 100psi across the board, to 150 across the board with the throttle wide open.

                  go check it again and hopefully feel stupid rather than bumbed out.

                    Comment

                    • Billy Ricks
                      #6
                      When you do a leakdown you can tell where the compression is going. Listen to the carbs, exhaust ports, and pull the oil filler cap off. Listen to each area to see where you hear the hiss. You may need a stethoscope. It should leak to zero, just a matter of how long it holds compression. Do your check with a warm engine too.

                        Comment

                        • Anonymous
                          #7
                          Holy COW!

                          You guys (gals too!) rock!

                          That's good news. I just re-checked the compression with the throttle open (didn't hold open the slides, just the throttle) and I'm sitting at 125 PSI! (at an engine temp of ~90F) .. I knew it had to be something stupid I was or was not doing.. It just started too well for 80 PSI.

                          Ahh.. no rebuild needed for now. Good news. Now to fix the suddenly developed clutch rattle. Thats another thread though. My guess it its the nut that came loose.

                            Comment

                            • Billy Ricks
                              #8
                              Don't feel bad, lots of people forget to open the throttle. When you consider your are just cranking for a stroke or two you need to make sure the cylinder gets a good fill.

                                Comment

                                • Rambler
                                  #9
                                  compressiom

                                  got 77 in #4 105 in 1 2 3, 750 gs has 77.000 miles iam going to change motor i have another 1 thats is good what should compression be on good motor ill will be checking it cold,, the one iam going to put in, but i will check it cold beczause it has no carbs on it what should i get for compression if motor is good , it been 20 years the last time i work on bikes, iam old now and forgot alot thanks :?

                                    Comment

                                    • dgeorge
                                      #10
                                      iam old now and forgot alot thanks
                                      Hey I resemble that remark. I'm not sure on the 750's but my 1100 is supposed to run about 140 with a service limit of 110 I think.

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