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1978 CB750 Surging Idle?

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    #1

    1978 CB750 Surging Idle?

    Sometimes she idles at 4k, sometimes she barely idles at all and I have to keep on the throttle to stay running. Otherwise she runs great. No problems except for the tricky idle. Its not really hindering my ride, I just reach down and turn the idle screw in till she sounds right... and then reach down and back it out again when it idles too high

    I've heard that the carbs for this bike need to be absolutely perfectly tuned or else it does have a surging idle, and that for an easier idle tuning I could swap up to CV in the 1980 bike. Is this true?

    Cleaned the carbs to Bass' specifications, replaced my airboots, airbox, filter, o-rings, and intake gasket thingies. Replaced my points too.

    Ideas? Questions? Comments?
    Tags: None
    #2
    Hi,

    How are the air and fuel screws set? Have the carbs been vacuum synchronized? Are you positive there are no air leaks in the air intake system?


    Thank you for your indulgence,

    BassCliff

    Comment

      #3
      Originally posted by ZakDanger View Post
      Sometimes she idles at 4k, sometimes she barely idles at all and I have to keep on the throttle to stay running. Otherwise she runs great. No problems except for the tricky idle. Its not really hindering my ride, I just reach down and turn the idle screw in till she sounds right... and then reach down and back it out again when it idles too high

      I've heard that the carbs for this bike need to be absolutely perfectly tuned or else it does have a surging idle, and that for an easier idle tuning I could swap up to CV in the 1980 bike. Is this true?

      Cleaned the carbs to Bass' specifications, replaced my airboots, airbox, filter, o-rings, and intake gasket thingies. Replaced my points too.

      Ideas? Questions? Comments?

      CB750's are crap. Never liked those bikes. The design was pretty much state of the art in 1969 when they first came out, but by 1978 they were the worst bikes in their class.

      Get yourself a proper GS750 Suzuki. Superior bike in almost all ways. They don't have problems as you note as long as all the maintenance is up to date and the carbs properly synced and tuned.
      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

        #4
        Ah nuts. I meant GS750. Was talking to a friend about his CB at the time I wrote this post. Forgiveness please.

        Comment

          #5
          Originally posted by BassCliff View Post
          Hi,

          How are the air and fuel screws set? Have the carbs been vacuum synchronized? Are you positive there are no air leaks in the air intake system?


          Thank you for your indulgence,

          BassCliff

          The air and fuel screws are set to factory, the carbs have been vacuum synced, and ...er... pretty positive there's no leaks?

          The idle doesn't really move around while its idling. I'll start it up, it'll idle low and die out, i keep it going, take it off the choke, and then it'll idle high, then i'll ride and eventually it'll start idling low almost dying again.

          Comment

            #6
            Hi,

            Yeah, I figured the "CB" was a typo.

            Have you tested the petcock? Are you using an inline fuel filter?


            Thank you for your indulgence,

            BassCliff

            Comment

              #7
              Originally posted by BassCliff View Post
              Hi,

              Yeah, I figured the "CB" was a typo.

              Have you tested the petcock? Are you using an inline fuel filter?

              BassCliff

              Ugh. Serves me right for trying to do more than one thing at twice.

              I did not test the petcock... I have had the tank off many times throughout my time fixing it up, ON and RES both do not allow fuel out. I do not have a fuel filter on there. How does one test the petcock?

              Comment

                #8
                I had problems like this before. My issue was a gummy slide. The slide would stick in an open position periodically, so I'd back out the idle to correct. At the next stop, the idle was so low that the bike would struggle to maintain an idle.

                To check this, try idling the bike on the center stand with the carb tops off. Run a fan over the engine. Rev the bike a bit and see if it hangs. If so, gently press on the slides to see if that corrects the idle. Note which ones stick. If any slides are sticking, you should pull the carbs apart and clean the hell out of the slides and slide openings in the carbs.

                HTH

                Comment

                  #9
                  some one will correct me if im wrong. take a vacuum pump and put a fuel line on the petcock and in a clear glass jar when you put vacumm to the petcock with it in the on or reserve positions fuel should come out when you release vacuum it should stop the petcock should always flow when in prime position if the petcock is good check your vacuum line for leaks

                  Comment

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