Followup: re electronic ignitions for BMW airheads

Discussion in 'Motorcycle Technical Discussion' started by Rob Kleinschmidt, Mar 15, 2007.

  1. A week or two back, I'd asked about aftermarket all-electronic
    ignitions for airhead BMWs, particularly those with ignition
    canisters.

    At that time, the mechanical advance was screwing up big time
    and I'd been assured they couldn't be repaired. I was looking
    at all electronic ignitions with electronic advance as an alternative
    to replacing the very pricey BMW mechanical unit.

    I finally managed to rehabilitate the original mechanical advance.
    While BMW only sells the complete Hall cannister at ~ $440
    a pop, Motobins in the U.K. sells springs for a couple bucks
    each.

    For '79 on BMWs, electronic ignition options seem to be
    Omega (requires stock alternator) or Boyer (points canisters
    only, points can would fit all '79 on bikes).

    I decided that since I'm thinking of dual plugging the bike
    anyway, electronic advance would be of minimal benefit
    as the advance would be greatly reduced by dual plugs.
    With the mechanical advance working OK, I'll live with
    it for now and think hard about a dual plugging project.

    Thanks to all for good ideas.
     
    Rob Kleinschmidt, Mar 15, 2007
    #1
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  2. As you say, dual plugging allows the bike to run higher compression
    and/or lower octane gas as there is faster flame propagation.

    Because of the reduced time needed for flame propagation, people
    often recommend either retarding the timing a few degrees
    and better yet, limiting the advance mechanism. This is sometimes
    accomplished by adding a few layers of heat shrink tubing to
    the stop post to limit the throw of the weights.

    In any case, it would seem that with better flame propagation, trick
    ignition timing perhaps becomes less of a win than it would be
    with a single plugged engine. This is per some reading on various
    airhead sites.
     
    Rob Kleinschmidt, Mar 15, 2007
    #2
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  3. Motobins is great. I got new out of production Koni rear shocks
    from them, not available here. I've talked to many guys over
    the years and the general consensus is that dual plugs were
    and are helpful only if you are suffering from pinging due to
    the lower octane gas than the combustion chamber was designed
    for. I'm not sure what you mean by the electronic advance being
    greatly reduced. The retarding and advancing is adjustable like the
    mechanical device by rotating the plate.
     
    Fletis Humplebacker, Mar 15, 2007
    #3

  4. Seems like that would wear pretty quickly. I'm sure the airhead gurus
    have tackled that one by now but I think some guys probably
    created more problems than they solved. I don't know why low
    RPM performance was an issue unless perhaps they spend a lot of
    time in city traffic. I would look at the info for your particular year
    and model. BMW long ago addressed the octane issue
    with their combustion chamber design. My '78 /7 heads had no
    problem with pinging, nor did/does my '84 RS ones and I use
    low octane gas.
     
    Fletis Humplebacker, Mar 16, 2007
    #4
  5. It's not particularly subject to wear. It makes contact with a
    nylon pad to act as a stop when the unit reaches full advance.
    No turning action or friction.
    The GS is speced to run 91 octane. I used to run 89 but went
    back to 91 when the problems with the advance unit started.

    A GS can spend lots of time crawling up dirt trails in the hot
    sun, so it's a different breed of cat than a bike dedicated to
    highway use. It does fine on highways too, but I want to be
    able to crawl along in first for long periods of time on hot
    summer days. I'd also like to not have to worry about finding
    crap gas at single pump stations way out in the boonies.
     
    Rob Kleinschmidt, Mar 16, 2007
    #5
  6. I thought it wasn't at full advance. I never had a can type unit but
    the metal posts were the limit of the weights' travel. If you mean
    putting heat shrink on the posts that the weights hit I don't see
    why it wouldn't wear through. Aren't the weights hitting the plastic
    repeatedly? Seems like a metal sleeve could be fitted. One thing
    to watch for is the dreaded cam shaft end nut kaput gemachen.

    A lot of guys prefer the GS and never even leave the road. Nice all
    around machine.
     
    Fletis Humplebacker, Mar 16, 2007
    #6
  7. I meant putting heat shrink on the posts which limit the
    travel of the weights. Dunno about "hitting" the weights
    repeatedly. This would be caused by rapidly cycling
    the engine between 1000 and 3500 RPM ?

    People who have made this mod report no problems. A
    metal sleeve could be fitted but the metal that it fits on isn't
    round, so the machining might be messy.

    Cam shaft end nut kaput gemachen isn't an issue
    on canister models. The canister is driven by an
    arrangement almost like a U joint that engages with
    a slot in the end of the cam. No nut to worry about.
    worry about. From what I understand, the points end
    of the camshaft could be a problem area on earlier
    bikes.
     
    Rob Kleinschmidt, Mar 16, 2007
    #7

  8. Yes, every time the engine revs up. The posts limit the advance
    travel so the weights do hit them each time.

    Mine were small and round so it is a different animal altogether
    then.

    Yes, they broke off rather easily. Over torquing was the usual
    MO but in my case it was under torquing and it loosened, causing
    the weights to actually bend the dinky shaft. It broke when I had
    it almost perfectly straightened. Enter the Boyer system that
    didn't use the weights. It was cheaper than a cam shaft plus install.
     
    Fletis Humplebacker, Mar 18, 2007
    #8
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