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  • #1

    Dies over 1/4 throttle

    I'm helping a friend with the '79 Special he bought. It wasn't running on a cylinder, so we took the carbs apart and cleaned them really well (i'm pretty good at it) Put it back together, and after knocking the cobwebs out of the engine and adjusting the idle, it idles really well. Runs on all cylinders.

    If you give it ~1/4 throttle (or less) it revs strong and smooth, up to ~7-8k RPM. If you give it ~1/3 throttle, it dies almost instantly.

    Other info: idle mixture screws all 1.5 turns out, the valves are not adjusted, carbs not synched, brand new intake boots, brand new k&n air filter, octy deleted, floats all at same height, no tears in diaphragms...

    I am baffled. I've had plenty of bikes that refused to idle (until fixed), but never one that refused to rev. Anybody have any ideas?

    Thanks!
    79 SF
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  • #2
    Float levels too low, it's running out of fuel when it kicks into the main jet system.

    You might remove the air box when you pull the carbs again. I always leave mine off until I get stuff working right so I don't have to fight it.

    It's easy to retweak the adjustments once you get things working right and put the air box back on.

    EDIT; You did remove the emulsion tubes from under the main jets and clean them real well also, right?
    Last edited by BA80; 05-12-2014, 05:23 PM.
    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.

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    • #3
      Originally posted by ARBakken View Post
      I'm helping a friend with the '79 Special he bought. It wasn't running on a cylinder, so we took the carbs apart and cleaned them really well (i'm pretty good at it) Put it back together, and after knocking the cobwebs out of the engine and adjusting the idle, it idles really well. Runs on all cylinders.

      If you give it ~1/4 throttle (or less) it revs strong and smooth, up to ~7-8k RPM. If you give it ~1/3 throttle, it dies almost instantly.

      Other info: idle mixture screws all 1.5 turns out, the valves are not adjusted, carbs not synched, brand new intake boots, brand new k&n air filter, octy deleted, floats all at same height, no tears in diaphragms...

      I am baffled. I've had plenty of bikes that refused to idle (until fixed), but never one that refused to rev. Anybody have any ideas?

      Thanks!
      How far down did you take the carbs for cleaning? Did you remove the slides so the emulsion tubes could be removed and cleaned?
      Might also pull the left side ignition cover and tug on every inch of those pick-up coil wires. Any insulation stretch, its got a broken wire. While there, make sure the mechanical advance unit rotates very freely. Make sure vacuum advance hose is hooked to brass nipple on #2 carb, not manifold nipple. When that's all known to be fine, set idle mixes and sync, or you'll just be guessin', chasin' likely several issues.
      81H Venturer1100 "The Bentley" (on steroids) 97 Yamaha YZ250(age reducer) 92 Honda ST1100 "Twisty"(touring rocket) Age is relative to the number of seconds counted 'airing' out an 85ft. table-top.

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      • #4
        Good point Brant. I fought a carb problem for a long time only to find out it was pick up coil wires.
        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

        • #5
          Hmmm, did not remove the emulsion tube, never seen one in a carb before and didn't know it came out. I did hose it down really good with carb cleaner and compressed air. I'll take a look at the carb section and see if I can figure out how to remove them. Thanks!
          79 SF

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          • #6
            I just figured it out! Every bike i've ever cleaned the carbs on, the emulsion tube screwed out. I saw the brass piece in there, but just assumed since there was no fastener on it that it must not come out. That's gotta be it!
            79 SF

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            • #7
              Originally posted by ARBakken View Post
              I just figured it out! Every bike i've ever cleaned the carbs on, the emulsion tube screwed out. I saw the brass piece in there, but just assumed since there was no fastener on it that it must not come out. That's gotta be it!
              Removing main jet will allow it to slide out(towards vacuum assembly you removed). Slot in casting allows it to go in just one way. Don't lose the washers under main jets!
              Do NOT use metal wire or similar to clean any of the brass pieces. Use a toothpic to ream clean the side air-bleed holes of the emulson jets. Same goes for pilot jets
              Keep all pieces from each carb WITH that particualr carb. No mixin' and matchin' here.

              Precisely check and re-set if necessary ALL eight floats. Float/fuel levels ARE critical to all four carbs performing as close as you can get the same.
              81H Venturer1100 "The Bentley" (on steroids) 97 Yamaha YZ250(age reducer) 92 Honda ST1100 "Twisty"(touring rocket) Age is relative to the number of seconds counted 'airing' out an 85ft. table-top.

              Comment

              • #8
                You can use a wire to clean jets if it's of a softer metal than the jets. I've used strands of copper wire to clean a jet without any problems...

                Geezer
                Hi my name is Tony and I'm a bikeoholic.

                The old gray biker ain't what he used to be.

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                • #9
                  Make sure you bench synch them too before you put them back on.
                  Jeff
                  77 XS750 2D completely stock
                  79 SF XS1100 "Picky" stock with harley mufflers

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