Carb / idle problem on a Honda CB350

Discussion in 'Motorbike Technical Discussion' started by asa, Jun 29, 2006.

  1. asa

    asa Guest

    I have a '72 Honda CB 350 twin. It sat for 10 years before I bought it
    so the tank was quite rusted and gumming things up. I cleaned and
    coated the tank, replaced the fuel lines, and rebuilt the carbs. It
    runs now but the idle sometimes fluctuates and the right cylinder is
    running rich (it's smoking and blackening the plug). I have the mixture
    screw screwed in as far as it will go but it's still rich. I set the
    idle and fuel mixture using a tachometer as the clymer manual
    suggested. The idle is set at 1200 but sometime is will speed up to
    3000 or drop down low so the engine dies.

    Any ideas as to why one side is rich and the idle is irregular? I've
    tried everything I can think of.

    asa, Jun 29, 2006
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  2. asa

    FB Guest

    Are you sure whether the idle mixture screw is actually a fuel/air
    MIXTURE screw, or is it an idle AIR screw on that model? I can't find
    an online parts diagram for CB-350's.

    There are different ways of doing the idle mixture VS idle air on a
    constant vacuum carburetor.

    The idle MIXTURE circuit sucks air through the pilot air jet and
    gasoline through the idle jet. The gas and air MIX, and the idle
    mixture screw *actually* adjusts the flow of idle mixture, so turning
    it clockwise LEANS the mixture. Idle mixture screws would be on the
    downstream side of the throttle butterfly. They would be on top of the
    carb or underneath.

    OTOH, if you have an idle AIR screw, it's upstream of the throttle
    butterflies and it adjusts the amount of AIR mixing with the gasoline.
    So turning the idle AIR screw clockwise richens the idle mixture.

    An idle AIR screw would be on the SIDE of the carburetor, near the
    inlet bell.

    Then there is the issue of transition ports. Transition ports will be
    just downstream of the throttle butterfly. If the main outlet port of a
    carburetor with an idle mixture screw
    is plugged up, the engine will idle too slow and the owner will turn
    the idle speed up.

    This uncovers the transition ports and the engine will idle much too
    fast when hot and stall when cold.
    Sounds like plugged a idle jet and idle passages and the adjustable
    port is plugged and the idle speed is set too high, uncovering the
    transition ports.

    Don't worry if you have a hard time figuring that one out. It took me
    about 30 years to realize exactly what was going on with transition

    The other alternative to idle mixture or idle air problems is a stuck
    float valve on the one carburetor. That would raise the fuel level in
    the float bowl making it too easy for the engine vacuum to lift excess
    fuelinto the idle circuit.
    FB, Jun 29, 2006
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  3. asa

    asa Guest

    I'm not sure whether the screw is air or fuel. It is referred to in the
    manual as the "idle mixture screw". I posted the exploded view from the
    manual here if it helps:

    I believe turning it clockwise makes it lean because it runs poorer and
    smokes more as I unscrew it.

    I put in new float valves when I rebuilt them. Could it stick even if
    it's brand new?

    asa, Jun 29, 2006
  4. asa

    FB Guest

    Good man! Can you post more pages from the manual so we can read what
    it says about adjusting the idle mixture? Many owners can benefit from
    whatever text and pictures to can post.

    Looking at the idle mixture screw (# 9), I notice that it is horizontal
    and that there is no tiny o-ring associated with it. Idle MIXTURE
    screws need this o-ring because there is liquid flowing past them. Idle
    AIR screws don't have the o-ring because all they can do is leak a
    little extra AIR around the threads. This tiny gap usually gets sealed
    evaporated gasoline that turns to varnish.

    My best guess is that # 9 is an idle AIR screw and that turning it
    clockwise richens the idle mixture by restricting air flow.
    Maybe turning the screw counterclockwise is allowing more air into the
    idle mixture and the excess idle mixture will burn out after a few

    Also, remember what I said about the transition ports. They wouldn't
    show on that carb drawing because the ports are just holes. If you
    richen up the idle mixture too much, and the engine idle speed slows
    down, the natural tendancy is to turn the idle speed back up, not
    realizing that the extra butterfly opening is causing the engine to
    suck extra mixture through the transition ports.

    The transition ports flow extra mixture for acceleration as the
    throttle is just cracked open.

    This extra mixture isn't needed for idling, so the idle RPM runs away,
    or the spark plugs foul.
    If there is gum and varnish on the float valve, it will stick. Float
    valves are often square or even triangular, in order to reduce the area
    of contact with the cylindrical sides of the float valve seat and to
    enhance fuel flow.

    A little bit of varnish from evaporated gasoline will cause the float
    valves to be "glued" in place, and gasoline can't flow past a stuck
    valve (or too much gasoline flows when the valve is stuck in the open
    FB, Jun 29, 2006
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