What order? synchronize/air mixture adjustment

Discussion in 'Motorbike Technical Discussion' started by Brent, Aug 10, 2004.

  1. Brent

    Brent Guest

    I'm wondering what is considered the ideal order in which to do carb
    adjustments? I need to synchronize my carbs, and I also need to adjust the
    idle air mixture.on my '82 CB900C (inline 4, keihin carbs)

    Follow-up question: I will be using mercury carbsticks to do the synchro;
    I've heard they also are a great tool for adjusting the idle mixture; adjust
    for highest vacuum, as being roughly the equivalent of highest rpm. Any
    thoughts on this?
    Thanks
    Brent
     
    Brent, Aug 10, 2004
    #1
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  2. Brent

    Mark Olson Guest

    The first thing you need to do is verify that the valve clearances are
    correct, before you do any carb synching or idle mixture adjusting.
    This is vitally important.

    I can't help you with the mixture setting procedure, other than to
    say I doubt it matters which carb goes first when you are doing the
    50rpm idle drop routine as spec'ed in the factory service manual.

    You are probably right that the highest vacuum will occur at the
    highest rpm, but the effect is going to be pretty small in the region
    of interest that you will be adjusting to, which is why the procedure
    in the manual says you cannot use the bike's tach to perform the
    adjustment, you need to use a tach with higher precision.

    I never managed to complete the idle drop adjustment by the book on
    my CB900C. I just picked a fixed number of turns on the adjuster
    screw that I was happy with and set them all to the same number.

    I don't have my manual any more, but I would be surprised if the
    order of precedence was anything other than synch first, then set the
    idle mixture. Of course you should set the mixture screws to their
    initial settings per the book before synching, too.
     
    Mark Olson, Aug 10, 2004
    #2
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  3. Brent

    Brent Guest

    Mark:
    Thanks for the comments. I'm sure the SV650 has made you forget all about
    your time with the lumbering beast of a 900C.
    Did the valves already. All are now bang on factory spec. Also, new plugs
    and new airfilter.
    My question was poorly worded. sorry. I wasn't concerned about which order
    to adjust the idle screws, but rather whether to synchronize first, then do
    the idle mixture screws, or vice versa.
    I know what you mean about the idle drop method as outlined in the manual.
    I tend to get bored halfway through and just wander off. Thought I'd tough
    it out this time. I'm shimming the jet needles with one washer each before
    I do any of these adjustments, and I imagine there's a chance that this will
    affect carburation at a number of throttle openings. (non-adjustable
    needles, as you remember) The engine has only 22,000 miles on it, and
    appears to be running just a tad lean at all rpms (the plugs are whiter than
    I am used to, and I'm getting about 2" of light blueing on the headers).
    Thanks for the help
    Brent
     
    Brent, Aug 10, 2004
    #3
  4. Brent

    Tostada Guest

    Sorry to jump in late here:
    It's been awhile since I synched carbs on my old Honda 550, but from
    what I can remember, you want to have the valves and mixture right to
    start. The synching mostly affects off-idle and low-speed, since this is
    where the slides are just opening and small differences have a larger
    percentage effect. At high speed, synch doesn't make much difference, so
    your plug color concern is a separate issue. You start with the slides
    set with the recommended opening at idle (can be measured with a drill
    bit of specified diameter). You'll find that the cylinders probably have
    different vacuum values, since they each pull differently and the
    manifold paths are not the same. You adjust to try and get them to the
    same vacuum level at idle, and then see how they respond to throttle.
    You want them to all respond about the same as you blip up from idle.

    The thing is, this is one of those 'tuning' issues: everything affects
    everything else, so when you change one carb, you'll see the values for
    the other carbs change. So you may have to go back and change them too,
    which then affects the first one you changed! I forget what bike you
    have, but the more carbs there are, the harder it is to keep track, but
    the better the results.

    So then your idle speed changes, and you have to readjust it, and then
    go back to synching. It's an art, like wheel-building, or differential
    equations.

    I'm probably mis-remembering something, but my main point is that it's
    tuning--everything affects everything else, so you optimise for the best
    overall result. Give yourself lots of time to do the job, be rested, and
    keep track with notes.

    Hope this helps.
     
    Tostada, Aug 10, 2004
    #4
  5. Brent

    Brent Guest

    Hey, thanks for the reply. I'll give it a try in that order. And thanks for
    the hint on going for the smallest setting.
    Yes, I was aware that the synch screws (locknut and screw) is not the same
    as the idle mixture screws.
    No need to apologize for directing a fellow to the manual. I have the Honda
    Factory manual, the Clymer and the Haynes. Gotta get my money out of them
    somehow.
    Brent
     
    Brent, Aug 24, 2004
    #5
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