R100 tuning

Discussion in 'Airheads' started by Indiesol, Oct 4, 2012.

  1. Indiesol

    Indiesol Putz

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    I recently rebuilt the carbs on my '81 R100. I used Bing repair kits, watched the DVD during the rebuild and used the Bing manual and a homemade manometer to sync and adjust. Needles are at position #2 (from the top).

    After putting the carbs back on the bike, as per the manual, I backed the idle adjustment screws off the levers, and while my friend held the throttle at around 1200rpms I used the vacuum gauge to set the throttle cable free play at the carbs. Then, also using the vacuum gauge, I set the idle to 1000rpms. Third, I tightened the idle mixture screws until lightly seated and then backed them out 1 1/4 turns and went for a tuning ride.

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    The Bing manual states that, from a standing start, there will be a bog (which there was), and to turn the idle mixture screws out (CCW) a "notch". It said to repeat this process until the bog, which I understood to mean from a standing start, is gone.

    I did this, and from a standing start, there is no bogging down, or if there is, it's so light that I don't notice it. However, when cruising, it seems like the throttle is not as responsive in the off throttle - 1/4 throttle department. It's alsmost like a lag rather than a bog. Once I get past a certain amount of throttle, the bike pulls hard, which I'm thinking means the bike is completely off the idle circuit. Should I continue to make small CCW adjustments on the idle mixture screws until the problem goes away? How, aside from fouled plugs, would I know if I've gone too rich?

    Thanks for the help!

    EDIT: The float height appeared to be set properly on both carbs when I pulled them apart, and I checked them before reassembly and they were good.
    #1
  2. Uncle Pollo

    Uncle Pollo Long timer

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    hard to tell ... how was it behaving before?

    The bog somewhere has to be rich condition ... my wild ass guess
    #2
  3. Wirespokes

    Wirespokes Beemerholics Anonymous

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    That seems like an odd way to set idle mixture.

    What's always worked for me was to start somewhere around the factory setting - something like 3/4 turn out on the mixture screws. Doesn't really matter where exactly, since they'll get adjusted later.

    Turn the idle speed/butterfly screws in a turn from just touching.

    Once the bike's fully warmed up (20 mile ride), turn the mixture screw in till it bogs, then out till it bogs. About half way between the two points is the best setting.

    Sometimes the idle will go much higher as you adjust - so bring it back down since it will affect the mixture adjustment. Play with it back and forth and it'll settle down and idle stably with a thump thump thump even idle.

    Once you've got that, raise the RPM if you want, but I like to leave it there.
    #3
  4. Indiesol

    Indiesol Putz

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    Thanks to both of you for your input, and hey Spokes! I really value your opinion, so I find it interesting that you thought it a strange way to adjust the idle mixture. The Bing manual came very highly recommended. I did read several online postings outlining methods similar to yours though, so I can always give that a go too.

    I did forget to mention one step in the process, where before going for the tuning ride you adjust the idle mixture screws inward until the bike is about to die from being too lean. I did that part too. But upon re-reading the Bing instructions last night, I think I may have an indication of where I screwed up. After going for the tuning ride the Bing manual suggests reinstalling the vacuum gauge and clearing up any mishaps. When I got back from my tuning ride I was so excited about getting it running pretty decently that I just sort of readjusted the idle by hand and called it good. I think I need to redo the process.

    At least now I have a second method to attempt if I can't get things a bit better using theirs.
    #4
  5. supershaft

    supershaft because I can

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    I have a Bing manual but I use it just to see what parts are available. I wouldn't recommend a lot of their tuning advise. Both their methods and recommended jet sizes leave a lot of room for improvement.

    Mixture screw adjustment is the hardest to master from what I can tell. Adjust the screws for fastest idle and then readjust your idle speed using a manometer. Then readjust your mixture screw again for fastest idle. Do that back and forth until fastest idle with the mixture screw is the idle speed you want. Now screw each screw in till it stumbles and then back out till it stumbles and note the turns as if the the screw slot was a clock hand. The stumble point is always much clearer on screwing it in. But the screw half way between the stumble points which is usually exactly where it was for fastest idle. Think it's a bit lean there? They often are. Turn the screw back out one hour. If your screws have different turns between stumble points and they often do, compensate. For instance, if one carb has one turn between stumbling points and one carb has one half turn. Turn the one screw one hour back out and the other one half an hour back out in order to richen both carbs the same. Good luck!
    #5
  6. Indiesol

    Indiesol Putz

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    Thanks all. Just wanted you guys to know I got it nailed down using your help.
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  7. supershaft

    supershaft because I can

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    :clap
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  8. Airhead Wrangler

    Airhead Wrangler Long timer

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    "I love it when a plan comes together."

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