Tuning an airhead - science or art? Help appreciated

Discussion in 'Airheads' started by bikecat, Oct 5, 2010.

  1. supershaft

    supershaft because I can

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    Correct jetting is way leaner than most people think. In actuality correct jetting isn't lean at all and what most people think is correct is WAY too rich. Bikes run pretty good way too rich except for making less hp, less mileage, and more carbon. I jet my mains like you jet your idle: just rich of stumbling. You can gain numerous hp doing that compared to many stock jetting setups. Nevertheless, idle and to a lesser degree midrange most always needs to be jetted a bit richer than optimum. Why the sacrifice? It's in order to better deal with the HUGE differences in vacuum and venturi size from cracking the throttle open. That a bit rich sacrifice is mostly to get through that just off idle lean spot that results from the big vacuum drop from opening the throttle. Starting that massive transition a bit on the rich side helps get through the lean times. Nevertheless, there is no need to go over board. More than just enough is too much!
    #21
  2. bikecat

    bikecat Long timer

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    Tinkered with the bike this weekend. The needles from motobins came and they were the ones for the 40mm Bings. Sigh, ...... cleaned up the old needles and re-installed them. Will go after new 32mm Bings' needles later.

    Surprisingly with these needles and stock BMW settings there's back-firing and the bike was unable to hold any idle even when warm. Rpm was like 200.

    Followed the wisdom provided, managed to tune the bike fairly decently, now I appreciate the "stumbling" points for the mixture tweaking. I did the off-idle adjustments for 2,500 rpm and will try Snowbum's recommended 1,300 and 1,500 next time around.

    The only area that I need more info and practice on will be the throttle cable adjustments; kinda difficult to ensure that both will engage simultaneously with the twist of the throttle. However, as things are now, I am really a happy puppy. :) Bikes runs really well, pulls hard and now I have to monitor the plugs and fuel consumption to see what they say.

    Thanks to all the assistance and wisdom provided, really appreciate this.

    Cheers
    #22
  3. supershaft

    supershaft because I can

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    are
    I hope I helped as well bikecat. I always suggest looking up your own PN's before ordering parts. It's easy on the idiotnet!

    If your original jet needles were worn in the area that they do wear right above the start of the taper for about 10mm, you need new needle jets as well. The needle wears itself out wearing out the jet. Does that make sense?

    Don't get me wrong, I look up info on Snowbum's and Duane's sites sometimes but I know how to weed through the misinformation. There are TONS of it on both sites! I do agree with Snowbum about syncing the cables right off idle. With a properly set up manometer, you can watch the cables move around a bit under the tension of opening up the throttles. Just after the cables settle in is where I sync them. Remember that tightening down the lock nuts changes everything. Sync accordingly.

    IMO, chasing some "cruise" rpm and load for sync is impossible. The load (manifold pressure) changes everything and it is always varying greatly.
    #23
  4. igormortis

    igormortis Cafe Reise

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    Interesting reading. It really does look like everybody finds their own way to getting these old bikes to run nicely.

    Has anyone welded O2 sensor bungs into the header pipes to monitor Air/Fuel ratios?
    #24
  5. bereahorn

    bereahorn Long timer

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    :ear
    #25
  6. bikecat

    bikecat Long timer

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    Hiya,

    Just an update. A pushrods seal leak became a piston rings overhaul operation upon the discovery of a broken ring. So hopefully after the installation of the rings my mileage will improve.

    As they say .... Things can only get better .... :clap
    #26