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gs750 surging, no idle when hot-Need help quickly!

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    #1

    gs750 surging, no idle when hot-Need help quickly!

    Hope you guys can help. My shop is trying, but I don't know if we have enough time before my long planned trip to Laguna Seca World Superbikes this weekend (2 days from now). My 1980750 starts easily and revs beautifully for the first 1/2 hour or so, then it starts to surge between 3500-4500 and stalls whenever I come to a stop (no idle). My mechanic to a look at it and riched out the mixture, and it rode better, then started the same thing again. A bit nervewracking riding up the busy Grade here between big rigs and itn starts bogging when throttle is steady and over 2/3 open. Even downshifting doesn't help. I thought I'd have to coast to the non-existent shoulder, then it kicked in a little. Woohoo, adrenaline rush! Ambient temp here is 105F. Am wondering if ignition too hot?
    Mechanic checked for maniforld leaks (fine), tank was cleaned and Kreemed by last owner. Plugs just replaced-all 4old ones seemed ok (coffe&cream). When it bogs, even full throttle just keeps it flat, then it spontaneously kicks in for a short bit and slides my butt back on the seat from acceleration.Won't rev past 4700 when it's acting up, so I roll along at 60MPH
    Any ideas?
    I don't want to get stranded up in Monterey, but I really, really, really want to go on my bike. I refuse to travel 2 up on hubby's Seca all that way and I hate the idea of driving a car when I've been dreaming of lining up my bike on Cannery Row w/all the others. Opinions? Would you do the ride w/bike in this condition? HELP(I keep hoping.....)
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    #2
    From your description of the plug colors, I would say the mixtures are correct. Also, the way it runs when everything is normal indicates the mixtures are OK. Tank is cleaned, so probably not silt or clogged filters. In 105 deg temps, if the ignition was going to break down, I doubt it would take 30 minutes for it to reach that heat level. I think your throttle settings when it starts acting up are high ones....correct?
    So, Your engine is demanding more fuel than it can get.
    I can think of two things to try. When it starts to bog down, reducing the throttle should result in MORE steady power since the correct fuel air mixture could still be maintained.
    I think most likely is a petcock fuel flow problem. Are you using the 5/16" fuel lines and clamps on the lines? Also, you can run the bike in the prime setting (for the stock vacum operated petcock) But you will have to remember to switch it to the "run" position when shutting down the engine.
    Running in the prime position will bypass the diaphram function in the petcock and assuming the fuel tank has no debris, insure that you are getting full fuel flow to the float bowls. I would take it out for a run with it in the "prime" position and see if I could make it bog. If it still bogs, and reducing throttle setting smooths things out, then I would still
    think its a fuel flow problem. I have a pingel petcock on my bike and on occasion, I have forgotten to turn the fuel valve to on. I can ride about 400 yards on what is in the floatbowls and then it goes flat very much like your description. Turning the fuel on does not improve anything for about 20 seconds. With the engine running, it takes that long for the valve to flow enough fuel to fill the float bowls and get ahead of engine demand. I use clear fuel line on my petcock to carbs and I can see if there are bubbles and lack of fuel flow also.

    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

      #3
      Also, with fuel supply problems, the engine "goes flat" and loses power usually gradually. Youre probably hearing a sound something like...Buuuuuh ??

      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
        sounds like the vent in the gas cap may be clogged and it is building up a vacume in the tank, preventing gas from flowing.
        or posibly a problem with the automatic petcock (gas valve), have you tried switching over to "prime" when it starts acting up.

        do you get alot of clattering/detonation when it starts to act up? this would indicate overheating.

        I would recomend that you put a spare key in the gas cap and take it for a ride and when it starts to act up imediatly pull off the road and remove the gas cap. if it is difficult to remove the cap and you get a whoose of air getting sucked into the tank you have found the problem and it will need to be disasembled and cleaned.

        Comment

          #5
          You guys are GREAT!! I drove my bike down to my new mechanic and thought for sure it was going to take at least through the weekend to troubleshoot, but my new mechanic was exactly on your wavelength and found out that my carbs were disgustingly full of rust!!. The guy I bought it from said he cleaned the tank and Kreemed it, but I assumed that he at least cleaned the float bowls, because he said he worked on them. OOOPS! the shop slipped my bike to the front of their list and fixed it so I can actually ride my old boy to Laguna Seca tomorrow. Oh yeah, he said to not worry about settling up and just ride it to make sure all is well over the weekend and get in touch later.
          It runs so smoothly now that I havr to relearn how to ride it because it doesn't buck anymore or stall.
          Now, the only problem I have is that my butt keeps slipping back on the saddle every time I accelerate, even in 4th!! 8O 8O

          Comment

            #6
            I'm REALLY pleased that your bike is now running well and you can make your trip and enjoy it. :-) :-) As for the slipping of the seat problem, the solution is simple. You will need to purchase a set of chaps, a can of spray glue and two sheets of #80 grit sandpaper.
            Remove your jeans, don the chaps, turn around and have hubby spray the problem area with the glue. Apply sandpaper. Voila!, problem solved. :-)

            Have a great weekend.

            Earl


            Originally posted by Motomedic
            You guys are GREAT!! I drove my bike down to my new mechanic and thought for sure it was going to take at least through the weekend to troubleshoot, but my new mechanic was exactly on your wavelength and found out that my carbs were disgustingly full of rust!!. The guy I bought it from said he cleaned the tank and Kreemed it, but I assumed that he at least cleaned the float bowls, because he said he worked on them. OOOPS! the shop slipped my bike to the front of their list and fixed it so I can actually ride my old boy to Laguna Seca tomorrow. Oh yeah, he said to not worry about settling up and just ride it to make sure all is well over the weekend and get in touch later.
            It runs so smoothly now that I havr to relearn how to ride it because it doesn't buck anymore or stall.
            Now, the only problem I have is that my butt keeps slipping back on the saddle every time I accelerate, even in 4th!! 8O 8O
            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

              #7
              Thanks for your suggestion, but I think we're going to try Velcro instead of sndpaper-lasts longer(hahahaha)

              Comment

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