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1980 GSX400 - Still Stuck at 7k RPM After More Work

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    1980 GSX400 - Still Stuck at 7k RPM After More Work

    I’m still dealing with the same issue on my 1980 GSX400—it won’t rev past 7,000 RPM under load, and it’s hard to even get up to 100 km/h. This bike was my dad’s, and it sat for 15 years before I got it running again last August. Everything on it is stock, and here’s what’s been done so far:

    • Cleaned the carbs (6 times).
    • Synced the carbs.
    • Replaced throttle shaft seals, carb O-rings, and carb boot O-rings.
    • Replaced carb diaphragms (the old ones were destroyed), and both throttle slides come down smoothly.
    • Adjusted the valves.
    • Installed a Boyer electronic ignition with a new coil.
    • Installed an SH775 rectifier (charging at 14.52V).
    • Checked compression: 125 PSI on both cylinders.
    • Running the stock air filter.
    • Checked the exhaust: no blockages.
    • The petcock flows fuel good. (sucked on vacuum line and fuel comes out of the gas line and not the vacuum line)

    It feels like it hits a rev limiter at 7k RPM under load, but it will rev to 8,500 RPM when not under load (on the center stand). I’m fed up with this issue—any advice on what to try next?​
    Last edited by xxenzor; 03-08-2025, 01:28 AM.

    #2
    Does the centrifugal spark advance move freely?
    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

    Comment


      #3
      Originally posted by Nessism View Post
      Does the centrifugal spark advance move freely?
      Yeah the whole ignition works good. I followed the boyer instructions and it’s direct on the Advance mark at 5000 rpm

      Comment


        #4
        Did you grab the rotor, twist it against the spring, and release? Does it move freely?

        BTW, why did you clean the carbs 6 times? If you followed the tutorial available here, it shouldn't take more than ONE.
        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

        Comment


          #5
          Originally posted by Nessism View Post
          Did you grab the rotor, twist it against the spring, and release? Does it move freely?

          Yes it does. i’ve also cleaned it and the surface rust off


          BTW, why did you clean the carbs 6 times? If you followed the tutorial available here, it shouldn't take more than ONE.
          Ive only soaked it once but the 6 times means more if i’ve taken it off and sprayed the jets and holes to make sure it wasn’t clogging up

          Comment


            #6
            I know this sounds silly but is the air filter new?
            82 GSX1100SXZ Katana
            82 GSX750SZ Katana
            82 GS650GZ Katana

            Comment


              #7
              You never answered about the mechanical advance unit. Maybe the aftermarket ignition is binding up the advance unit.

              And cleaning the carbs 6 times means you didn't to it right the first time. Is the gas tank clean and rust free. And no, a filter is not a proper way to eliminate rust from entering the carbs.
              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

              Comment


                #8
                [QUOTE=Nessism;n1765680]You never answered about the mechanical advance unit. Maybe the aftermarket ignition is binding up the advance unit./QUOTE]

                To be fair, the xxenzor did say, " I followed the boyer instructions and it’s direct on the Advance mark at 5000 rpm"

                Boyer ignitions are (and I may be wrong), electronically controlled spark advance and the bob weights etc. are redundant. If the timing is correct at full advance as per the Boyer instructions (and the only way you could confirm that is by dynamic timing with a strobe). And assuming the correct Boyer ignition kit is being used (the correct full advance in degrees is set in the circuitry) if the advance aligns at the stated RPM, its sounds like the ignition is working correctly. ​​
                Last edited by KiwiAlfa156; 03-09-2025, 08:49 PM.
                82 GSX1100SXZ Katana
                82 GSX750SZ Katana
                82 GS650GZ Katana

                Comment


                  #9
                  Hmmm, how old is the Boyer unit? Was it new when you fitted it? Are there any identification numbers on the controller box? Do you have the kit number?
                  Last edited by KiwiAlfa156; 03-09-2025, 08:48 PM.
                  82 GSX1100SXZ Katana
                  82 GSX750SZ Katana
                  82 GS650GZ Katana

                  Comment


                    #10
                    Originally posted by KiwiAlfa156 View Post
                    Hmmm, how old is the Boyer unit? Was it new when you fitted it? Are there any identification numbers on the controller box? Do you have the kit number?
                    Brand New Boyer, I’ve talked to boyer when installing it to make sure everything was right. Kit 00291.

                    It’s weird cuz when it hits 8000-8500RPM not under load it just stays there as if it was the a rev limiter. And then when its under load it barely wants to get up to 7000rpm.

                    I soaked the air filter foam in degreaser and made sure it was super clean and then reoiled it

                    It seems like it would be air/fuel related but i’m not sure what else to try

                    Comment


                      #11
                      If I were you I would check to make sure the pin count on the timing change between cams is correct. Had the same problem on a new to me 850L and it took me a couple months to figure out that there was 21 pins between the cams not the correct 20 pins. When that happens the intake valve opens late and restricts the top end. Shouldn’t take you long to find out. Just a thought.

                      V
                      Gustov
                      80 GS 1100 LT, 83 1100 G "Scruffy"
                      81 GS 1000 G
                      79 GS 850 G
                      81 GS 850 L
                      83 GS 550 ES, 85 GS 550 ES
                      80 GS 550 L
                      86 450 Rebel, 70CL 70, Yamaha TTR125
                      2002 Honda 919
                      2004 Ural Gear up

                      Comment


                        #12
                        what do your plugs look like? maybe check your float height? sounds like maybe it's running lean up near wot?
                        1982 GS450 txz

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