balancing carbs on R90/6

Discussion in 'Motorbike Technical Discussion' started by gabe2004, Apr 28, 2006.

  1. gabe2004

    gabe2004 Guest

    Hi. I posted to this group a while ago about my R90/6. I have taken
    it out of the garage this year and amd determined to get it running
    well. It first it was very hard to start and when it did, ran on one
    cylinder. I cleaned both carbs and also replaced the air filter and
    it's running much better. However, there is a lag between when I open
    up the throttle and the engine responds. It's very noticible. I was
    thinking that the carbs might need to be balanced. I've never done it
    before. Any advice on the best way to do it?
    gabe2004, Apr 28, 2006
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  2. gabe2004

    John Johnson Guest

    check out the site. I distinctly recall seeing carb
    balancing techniques there.

    got it:

    THere are certainly other descriptions out there, but these are two that
    I've read which don't seem wacky or dangerous. HTH


    'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.
    John Johnson, Apr 28, 2006
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  3. Two basic techniques (bear with me if I'm missing an R90 peculiarity)

    1) Vacuum balance. You attach a vacuum gauge to a little port on
    the underside of the carb and fiddle with the idle adjustments
    until the two carbs are drawing equal vacuum. You can make
    a differential vacuum guage out of $5-10 worth of plastic tubing
    and drip irrigation widgets.

    2) Shorting the plugs. You cobble up an extension to the spark
    plug thread such that there's a length of exposed metal running
    between the plug and connector.Using an insulated screwdriver,
    you short out one plug at a time, observing the RPMs and
    adjusting the idle until the RPM for each cylinder matches.

    In either case, you probably want the idle richness screws about
    1/2 to 3/4 of a turn out from fully screwed in, then play with the idle
    speed screw on each carb. The richness adjustment screw is (I think)
    on the underside of your carb. The speed adjust would be on the top
    or side. I don't recall much about the exact Bing carb you have,
    so you'll need to examine it to find these screws.

    You probably want a Haynes and/or Clymer manual. I prefer having both
    so I can compare and contrast the instructions.

    I usually use plug shorting because it's easy and works fine for me.
    You can use an old spoke end or buy a 4mm tap and some 1/8"
    aluminum spacers to make the plug extensions. I also built a
    guage but don't use it much because it's more of a PITA to set up.

    Check out the airheads and ibmwr websites for some good tech articles
    on both techniques.
    Rob Kleinschmidt, Apr 28, 2006
  4. gabe2004

    OH- Guest


    I guess it is self evident but if the OP is a real newby it might
    be worth mentioning.
    After you get equal idle performance on both carbs, you need
    to make sure the throttles open up at exactly the same time
    when you twist the throttle grip. Either you eyeball it or you
    do renewed tests at very low throttle. You adjust the cable
    free play at each carb to get it right.
    Typical symptoms when this is wrong on a boxer is both lag
    and a sort of shudder at low throttle openings (the boxer
    effectively being a thumper).
    OH-, Apr 29, 2006
  5. gabe2004

    SAMMM Guest

    just my 2 cents...
    when i did the old superhawks, etc. i would disable one plug and get the
    best idle on that
    then i would do it to the other cylinder.
    when both cylinders were enabled, naturally, the idle was too high so
    from there i would move both idle SPEED (throttle openings) equally to
    the idle to the proper level.
    this had the effect of setting each cylinder with a slight load which
    produced a bit better
    just-off-idle characteristic.
    just another way.
    SAMMM, Apr 29, 2006
  6. gabe2004

    gabe2004 Guest

    Thanks for all the suggestions. I think I will give the shorted spark
    plug method a try and see how it goes.
    gabe2004, Apr 29, 2006
  7. gabe2004

    MadDogR75 Guest

    Lag between throttle opening and response sounds more like
    a hang-up in the movement in one or both of you carb pistons.
    Are you sure you got em CLEAN? Is the reassembly tight?
    Its OK to short a plug on the /6, (not so on later Cap. Ign. models).
    Try to identify the offending carb that way.

    Clymer and Haynes both recommend shorting plugs and feeling exhaust
    pressure at idle for balancing older air-heads.
    You have to set the idle high to keep her running on one jug and then
    back em off equally with both on line.

    MadDog -"R75/5 Forever!"
    MadDogR75, Apr 30, 2006
  8. gabe2004

    gabe2004 Guest

    I'll double check the carbs. Btw, there is no lag at higher rpms, only
    when idling.
    gabe2004, Apr 30, 2006
  9. gabe2004

    John Johnson Guest

    That can be hard on the coils. All that energy that otherwise would have
    generated a spark is now being dissipated in other ways, with some of it
    likely being reflected back to the coil, potentially causing it to

    'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.
    John Johnson, May 1, 2006
  10. Blowing a coil by shorting the spark sounds like a pile of
    crap. I think you're more likely to do damage pulling
    a plug wire.
    Rob Kleinschmidt, May 1, 2006
  11. gabe2004

    gabe2004 Guest

    Ok. I was able to quickly use the spark plug method to balance the
    carbs. I still have to adjust the throttle cables but the throttle lag
    is already better. What should I set the idle speed to? Right now
    it's about 1200 RPM.
    gabe2004, May 6, 2006
  12. gabe2004

    SAMMM Guest

    my guess is around 1000 rpm. what does the book say?
    i had a r100rt but removed the engine shortly and stuck in a
    small diesel so i'm not familiar with it.
    good luck, sam
    SAMMM, May 6, 2006
  13. Congratulations on your success.

    I usually go for about 1100. Depends on the bike. 1200 when
    it's really warm isn't bad at all. It's going to be sensitive to
    engine temperature. The R90 has a heavier flywheel so
    I might try for 1000.

    My morning commute takes me straight off the freeway to a long
    red light. Coming off the freeway, my idle may be as high as
    1200-1300 but is normally 1000-1100 by the time I get a green

    Also, if you haven't already done so, consider a careful search
    for air leaks in the manifolds and carbs. Any leak in the intake,
    carb or exhaust will cause the bike to run funny and fixing it
    will improve things tremendously.
    Rob Kleinschmidt, May 8, 2006
  14. gabe2004

    gabe2004 Guest

    I took it for a ride to day and it was running wonderfully. Engine was
    nice and smooth. I did notice that the idle had dropped to 900-1000
    and seemed to stay at that.

    Another quick question. The bike is pulling to the right a bit if I
    take my hands of the bars. I don't remember this happening before?
    Any ideas?
    gabe2004, May 9, 2006
  15. gabe2004

    SAMMM Guest

    first try to move weight from one bag to the other.
    my r100rt was quite sensitive to asymetric weight.

    if no joy:
    loosen the lower tube clamps on the tripletree,
    loosen the top fastenings may also need to loosen the
    jam nut at the top of the neck, (where the bearings would be adjusted.)
    you may be able to tweak the handle bars one direction or the
    other in relationship to the front wheel.. this may allow you to reset
    the balance of the bike.
    good luck, sammm
    SAMMM, May 9, 2006
  16. I agree with loosening and re-tightening the clamps.
    You might also take a look at the rear swingarm.

    The swingarm should be centered in the frame.
    I'll usually use an allen wrench as a guage to
    make sure there's equal space on each side.

    I won't try to quote the torque spec from memory,
    but it involves preloading then backing off and
    torquing down to a lighter value.

    On some models, there are also other spacers on
    one or both wheels.

    There's a parts CD floating around among airheads members
    which contains some good drawings. Haynes and Clymer are
    not too bad either, but should always be read a little skeptically.
    Rob Kleinschmidt, May 9, 2006
  17. gabe2004

    gabe2004 Guest

    Hey guys. I Happened to notice that there is a rubber bushing where
    the rear right shock absorber connects to the swing arm. It's almost
    completely worn which means that the shock absorber is lower by a
    couple of millimiters. Do you think could cause the bike to pull to
    the right?
    gabe2004, May 25, 2006
  18. I don't think that'd cause it to pull to the right but I think you
    ought to replace it anyway.

    Put the bike on the centerstand with the back wheel up in
    the air. Try to wiggle the rear swingarm and the rear wheel.
    If either of these will wiggle then you have a serious safety
    issue. Put the front wheel in the air and repeat the wiggle test.
    While it's on the centerstand you could use some string
    to check the front and rear wheel alignment.

    Note too that boxers will always want to lean to the
    right when you rev the engine. Probably more true on
    the R90 with the heavier flywheel. You can see this
    when you blip the throttle while stopped at a light.
    Rob Kleinschmidt, May 25, 2006
  19. gabe2004

    gabe2004 Guest

    Looks like i found a problem. I finnaly got the center stand and put
    the bike up on it. The back wheel did wiggle. It didnt't seem a lot
    but was noticble. I then did the string test and there was a 1 inch
    difference between the sides which would mean that the back wheel is
    half an inch to the left of the front wheel. Any ideas how to fix
    gabe2004, May 27, 2006
  20. I'm not that up on the R90, so I'd refer you to manuals. I believe
    that any two legged swing arm has bearings in the wheels
    which can go out.

    There could also be problems with the swing arm bearings.
    Is the play all at the wheel or is the swingarm sloppy as well ?

    Wherever your problem is, you'll need a parts manual, a bearing
    puller and probably a heat source (heat gun or torch). Identify
    the exact location of the play and consult your shop manual and
    parts CD to be sure you have the correct stack of spacers in place.

    Good writeup:

    Not sure if this is something you want to tackle or if you're
    better off handing it off to a shop. You want to be as gentle
    as possible removing and installing bearings into aluminum.
    You also want to be sure you never pull, push, tap, etc on
    anything but the outer bearing race when you install the new
    bearing. You can often use an old race to drive the new race
    into place.

    If you go to buy parts, it wouldn't hurt to bring the wheel along
    and see if the dealer had any advice. Where are you located ?
    Perhaps someone can recommend a good local shop.
    Rob Kleinschmidt, May 30, 2006
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