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Misfiring at highway speeds?

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    #1

    Misfiring at highway speeds?

    Hi,
    I have a small problem with my 85 Suzuki GSX750ES, similar to 83 GS750ESD/85 GS700ES. At constant speeds above 56mph/90kmh, the bike behaves a kind of jerky. In a way it seems as the power is fluctuating, approximately as a small misfire. There is no hearable popping and the tacho does not show any change in rpms. Anyway I have to compensate with the throttle to maintain constant speed. This is annoying. When accelerating or decelerating the bike runs smooth. At speeds below 56mph there is no problem, regardless rotational speed. It idles great (does not stall until 500rpm) and the plug readings are good. The carbs are cleaned and synchronized. The valves were adjusted last year.The intake air filter seems clean, but perhaps I should try to clean it one more time. I have sprayed carburetor clean on the manifold boots and the tube outlets from the air cleaner. The engine does not respond to this, so I do not suspect an intake leak in this area. My ignitor has been resoldered, and it is my suspect no.1. But if the ignitor is faulty, I do not understand why the problem does not also show at lower speeds. I would really appreciate help on this problem. Please, have any of you guys experienced something similar?
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    #2
    what kind of exhaust system are you running?
    I suspect you are a bit lean at that point in the RPM range and throttle position.
    it is a very common problem that many bikes have.
    shimming up the needles, or lowering the clip if t=you have adjustable needles will correct this.
    you may have to do some looking around to find them, but the shims are like washers but are very very thin, and it often takes several attepts to come up with the combination that works best.

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      #3
      The exhaust is actually stock. After my knowledge, I do not have adjustable needles. But perhaps I could shim them up. When I cleaned the carbs, I set the air screw to 2 and 1/8 turns out. As a compromise between the service manual 2 1/4 out and what has been suggested on this forum 1 1/2 - 2 turns out. Perhaps I should go to 2 1/4 to get even richer, but I was so satisfied with the plug readings....

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        #4
        I have had problems like this in the past and the ignitor was shot one time, a service manual should tell you how to check it. I've had several leaky plug wires, it will get worse as the RPM's go up. Go into a dark garage and rev the bike up, pay close attention to the area around the boots and anywhere a plug wire is close to metal to see if you have a spark jumping. This is a simple check and it may save you alot of grief.

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          #5
          My GSX750EF used to have an intermitant misfire at about 4-5000 rpm, sounded like it was hitting a rev limiter and it would lose power, then jerk back in.
          Turned out that the signal generator wires running on the exhaust were melted and occasioanly shorting, particularly at higher revs. This wiring was all wrapped in a heatshield sleeve so it wasnt really obvious at the side of the road!
          try pulling it out from under the engine and peeling back the heatsleeving - it wasnt reading bad when i did a continuity test, only a problem when it was wet or at high rpms.

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            #6
            At constant speeds above 56mph/90kmh, the bike behaves a kind of jerky. In a way it seems as the power is fluctuating, approximately as a small misfire.
            I won't comment on the ignitor issues, but this sounds like a lean surge to me. And the conditions you describe are perfect for finding one, small throttle opening and steady cruising. At that speed you will barely be off the pilot circuit and onto the needles. With stock jetting that is a great place to find a lean spot.

            I suspect you are a bit lean at that point in the RPM range and throttle position.
            it is a very common problem that many bikes have.
            shimming up the needles, or lowering the clip if t=you have adjustable needles will correct this.
            I agree. If you want to shop for small washers, Radio Shack is a good place and other stores like that. They often sell bags of assorted small washers for a couple of dollars. On my 1100E I have adjustable needles, but there is a washer under the E-clip as well and it measures 0.271"OD x 0.125" ID x 0.020" thick. I would bet you want a similar sized washer for your shims. Look for washers that are no thicker than 0.020", as you will be able to notice that much of a change on the OEM needles.

            Perhaps I should go to 2 1/4 to get even richer, but I was so satisfied with the plug readings....
            1/8 of a turn will not be noticeable on your plugs. I didn't notice much change until I started tuning in 1/2 turn increments. If you don't want to touch the needles, you can play with the mixture screws to compensate the overall mixture and find a compromise that works OK for you. It would be better to shim the needles, because that is really where the issue sounds to be, but I can understand not wanting to tear into your carbs, too.

            Personally, I would shim the needles and solve it that way, then not have an issue at all anymore.


            Mark

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              #7
              I have had that problem. The ignitor was was having a slow death. As it gets worse, it will cut out at lower speeds and finally, it will shut down on two cylinders.

              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.

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                #8
                Ok, thanks everyone! This forum is gold. I went to the local motorcycle junkyard and got to borrow an igniter, just to exclude it before I tamper with the carbs. It seemed to solve the problem. Anyway, the used CDI cost at least 156$ here in Norway, so I wondered if anyone have tried the "Hyperpak" sold at Ebay for 135$. In my ears a brand new unit with 12 months warranty for 135$ sounds better than the used suzuki for 156$. But if the Hyperpak is really crap...

                Regards,
                Sten

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