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GS750 Cylinders 1 and 3 only fire at 3500+ RPM

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    #1

    GS750 Cylinders 1 and 3 only fire at 3500+ RPM

    I have 1978 GS750. From the day I got it the plugs would foul VERY quickly. Within 40-50 miles. My friend(Who has worked on bikes for years) and I cleaned the carbs and checked the size of the main and pilots and the needle jets. The mains and pilots are both stock for my bike. The needle jets were changed for a smaller and more tapered size. We went back to the stock size, 5f21, and clip position. We put new spark plugs in. Within about 20-30 miles ALL the plugs were all fouled again. Cylinder 1 and 3 stopped firing UNTIL I got to about 3500 rpm and then they would kick in. We checked the coils and plug caps. The caps were old and didn't have the proper resistance. I upgraded to the Accel coils for my bike. We put new plugs in. We checked the compression on 1 and 3 and found that they were low(way under 100) dry and didn't go up when checked wet. I checked the valve clearances on ALL valves and found that 6 out of the 8 valves were too tight. I replace any shims that needed to be replaced. All valve clearances are within spec. We checked compression on ALL cylinders and they all were around 130-150psi . I put new plugs in. I got about 10-15 miles and the plugs on 1 and 3 were fouled AGAIN. 2 and 4 were almost the right color. And again cylinders 1 and 3 would only fire at higher RPM, 3500+. I thought it might be an electrical problem but the fact that NEW plugs temporarily fix the problem sorta points back to the carbs, I think. And yes the coil on the left is firing 1 and 4, the right 2 and 3.

    GS750 78'
    DynaS Ignition
    Stock Air Box and filter
    Stock main and pilots jets. Stock needle jets with stock e-clip position.
    NEW coils, wires, and caps.
    Carbs cleaned thoroughly. All jets cleaned. Float levels checked, set at 25mm. ALL the fuel bowls have fuel. ALL air mixture and fuel mixture screws adjusted and checked and rechecked.
    The voltage at the battery is about 12.5volts and goes up to about 13.5 at 3500 RPM.

    I spent way too much time and money to give up on this thing. :x
    ANYONE have any ideas or suggestions what could be wrong? Anything we missed? Help..............
    Tags: None
    #2
    You have covered most of it. The only thing I can think is if the choke plungers are leaking/ not closing completely and supplying fuel from the choke circuit

    Comment

      #3
      My first instinct is that your carbs are not set to the right intake vacuum.
      What are your carbs synch'd to...inHg? What are your fuel pilot and airscrew settings. Float levels set to ??

      Earl
      Komorebi-The light filtering through the trees.

      I would rather sit on a pumpkin and have it all to myself than be crowded on a velvet cushion. H.D.T.

      Comment

        #4
        OMG! I think that might be it! I don't remember having trouble with 1 and 3 UNTIL after we took the carbs off to clean them. I changed out the needle jets with stock ones. My friend came over to help clean them and I remember him adjusting ALL the slides ALL the way down. I distinctly remember at least one slide adjusted slightly different than the others. Will check the carb sync ASAP. THANKS

        Comment

          #5
          Hold on there! Matching the physical position of the slides does NOT synch the carbs. In fact, due to irregularities in the intakes, and length differences in induction passageways between cylinders, I can guarantee that setting all slides to the exact same static height will cause the carbs to be OUT of synch. You must use a mercury (or other) vacuum guage set to synch the carbs.
          It is vacuum levels that must be matched, not slide heights.

          Earl

          Originally posted by evldead09
          OMG! I think that might be it! I don't remember having trouble with 1 and 3 UNTIL after we took the carbs off to clean them. I changed out the needle jets with stock ones. My friend came over to help clean them and I remember him adjusting ALL the slides ALL the way down. I distinctly remember at least one slide adjusted slightly different than the others. Will check the carb sync ASAP. THANKS
          Komorebi-The light filtering through the trees.

          I would rather sit on a pumpkin and have it all to myself than be crowded on a velvet cushion. H.D.T.

          Comment

            #6
            Yes, I understand what your saying. That's my point EXACTLY. MY friend setting ALL the slides at the same height caused the carbs to be out of sync. B4 we cleaned and adjusted the carbs, I only had trouble with fouling plugs. It wasn't til after we cleaned and he adjusted them I had trouble with 1 and 3 NOT firing at lower RPM.

            I checked the vacuum on all the cylinders and YES, they were WAY out of sync! I don't have Carbtuner(YET) so I just got them sorta close( using a vacuum guage. It made ALL the difference in the world. I have ordered a Carbtuner from CycleRecycle and will sync them ALL up perfectly. But, at least NOW I know what the problem was. I never thought of the carb syncing until you mentioned it. A HUGE THANKS!

            Tom


            • Hold on there! Matching the physical position of the slides does NOT synch the carbs. In fact, due to irregularities in the intakes, and length differences in induction passageways between cylinders, I can guarantee that setting all slides to the exact same static height will cause the carbs to be OUT of synch. You must use a mercury (or other) vacuum guage set to synch the carbs.
              It is vacuum levels that must be matched, not slide heights.

              Earl

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