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Backfire and High revs

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  • #1

    Backfire and High revs

    Okay guys. Got that spark problem i had solved and pulled the carbs again to give em' another cleaning and bench sync. I built a manometer with the intention of doing a better sync once the bike was idling. Put the carbs back on, and started it up. It started without too much fuss and went into an idle very quickly. It was idling at 3100 RPM for a while, so I tried to adjust the idle screw a bit without too much luck. Never could get the idle down. Figured I would take it for a ride around the block to kinda see what my baseline performance was, but when I stood the bike up off the side stand, the idle rocketed up to 6000 with no sign of stopping. So i shut it down immediately. I tried to start it up again, which it did with a huge backfire out the left pipe, loud as a gun. It then immediately zoomed up to 6000 again, so I shut it down again. It did this any time I tried to start it. I am letting it cool now before I try anything else. Anyone know what is going on here?
    P.S. It took no choke to start it after it was warm, but it started with the huge backfire every time.
    Last edited by Aaron; 06-12-2011, 03:27 PM.
    1980 XS1100 SG Stock
    Tags: None
  • #2
    Year and model? (put it in your signature line so we don't have to ask)

    Did you bench sync?

    Where are your idle mixture screws set at?
    Nathan
    KD9ARL

    μολὼν λαβέ

    1978 XS1100E
    K&N Filter
    #45 pilot Jet, #137.5 Main Jet
    OEM Exhaust
    ATK Fork Brace
    LED Dash lights
    Ammeter, Oil Pressure, Oil Temp, and Volt Meters
    Green Monster Coils
    SS Brake Lines
    Vision 550 Auto Tensioner

    In any moment of decision the best thing you can do is the right thing, the next best thing is the wrong thing, and the worst thing you can do is nothing.

    Theodore Roosevelt

    Comment

    • #3
      First I'd check your throttle cable, make sure it's not somehow hanging up.
      2H7 (79) owned since '89
      3H3 owned since '06

      "If it ain't broke, modify it"

      ☮

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      • #4
        Throttle cable was my first guess, so I checked that and it does not hang up anywhere and the operation is smooth. Yes I bench synced. Not real sure where the idle mix screw are set, but they are all set so that the butterfly valves are open the same amount when throttle is closed. Should I screw them all the way in and then re-bench sync it?
        1980 XS1100 SG Stock

        Comment

        • #5
          How far open are the butterflies when at rest?

          The mix screws I am talking about are going to be on the front side of the carbs set down inside a tower. They may still have a plug covering them if you (or a po) did not remove it.
          Nathan
          KD9ARL

          μολὼν λαβέ

          1978 XS1100E
          K&N Filter
          #45 pilot Jet, #137.5 Main Jet
          OEM Exhaust
          ATK Fork Brace
          LED Dash lights
          Ammeter, Oil Pressure, Oil Temp, and Volt Meters
          Green Monster Coils
          SS Brake Lines
          Vision 550 Auto Tensioner

          In any moment of decision the best thing you can do is the right thing, the next best thing is the wrong thing, and the worst thing you can do is nothing.

          Theodore Roosevelt

          Comment

          • #6
            Sounds a LOT like a cable problem with the side stand thing.

            The other sounds like sync to me. Just saying.
            Greg

            Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid.”

            ― Albert Einstein

            80 SG Ol' Okie;79 engine & carbs w/pods, 45 pilots, 140 mains, Custom Mac 4 into 2 exhaust, ACCT,XS850 final drive,110/90/19 front tire,TKat fork brace, XS750 140 MPH speedometer, Vetter IV fairing, aftermarket hard bags and trunk, LG high back seat, XJ rear shocks.

            The list changes.

            Comment

            • #7
              The butterflies were only open enough to just let a bread tie wire slide under with resistance. After I put the carbs back on, I adjusted the idle screw, so I am not sure how open they are now. The mix screws you are talking about...inside a tower? Inside of the carbs? Any of the screws on the inside of the carbs are screwed all the way in as none of them seemed adjustable. I thought the idle mixture screws were on the outside of the carb? One main one on carb 3 and three others on top for adjusting 1, 2, and 4.
              1980 XS1100 SG Stock

              Comment

              • #8
                Nevermind guys, I got it figured out. Just had to be more persistent with my adjustments. I will try to get really stuck on something BEFORE i post next time.
                1980 XS1100 SG Stock

                Comment

                • #9
                  The screws you are talking about are the sync screws, the ones I am talking about are on the engine side of the carbs on the top side of the bell inside the tower that is there. I am guessing the plug is still there and it may look like there is nothing there due to that plug still being there.

                  What did you find the problem to be?
                  Nathan
                  KD9ARL

                  μολὼν λαβέ

                  1978 XS1100E
                  K&N Filter
                  #45 pilot Jet, #137.5 Main Jet
                  OEM Exhaust
                  ATK Fork Brace
                  LED Dash lights
                  Ammeter, Oil Pressure, Oil Temp, and Volt Meters
                  Green Monster Coils
                  SS Brake Lines
                  Vision 550 Auto Tensioner

                  In any moment of decision the best thing you can do is the right thing, the next best thing is the wrong thing, and the worst thing you can do is nothing.

                  Theodore Roosevelt

                  Comment

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