Carb Adjustment - slide guide

Discussion in 'Bay Area Bikers' started by Greg, Jul 14, 2007.

  1. Greg

    Greg Guest

    I 've just finished reinstalling my existing carbs (1993 900SS) with a
    used slide guide I purchased from eBay, hence the bike now idles
    correct. However a road test discovered that under full throttle there
    is very little power like 0 - 60 mph is 20 seconds! Well maybe not
    that bad.

    Proir the used slide guide the bike had very good excelleration
    however it backfired badly, due to a hole blown through the original
    slide guide.

    I can tell you that when I reinstalled the diaphragm it did not appear
    to seat correctly within the channel made for it. I just assume when I
    put the screws / diaphragm cover back on it seat itself in the right
    position. Any tips?

    Another possibility: could I have adjusted the throttle cables
    incorrectly? Is there a trick to adjusting them?

    Thats all the results I have to this point, any ideas? Anybody?

    Greg, Jul 14, 2007
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  2. Are they Mikuni CV carbs? I never heard of a Mikuni CV carb that had a
    removable "slide guide".

    Is it possible to install the slide backwards? Most CV carbs have
    assymetrical slides to prevent this from happening, but slides will
    have a bevel shape *cutaway* on the intake side tom enhance off idle

    Do you have a link to online Ducati parts fiches?
    It sounds like the vacuum slide isn't lifting, so the carb never goes
    on the main jet.

    However, Mikuni CV carbs have such problems with dirty idle jets and
    idle passages, that also keeps the engine from responding to the small
    throttle openings that uncover the acceleration transition ports under
    the lower edge of the butterflies.

    Other possibilities include incorrect plugged up air jet, wrong float
    height, stuck float valve, disconnected vacuum hoses, dirt in the fuel
    supply, etc.
    Any time you clean a carburetor with aerosol carb spray the neoprene
    rubber parts will swell up a little bit, but they return to their
    original size when they dry out.

    You can use a little grease or vaseline to make the lip of the
    diaphragm stick in the channel if you want.

    Some Mikuni CV carbs had a little rubber o-ring under the vacuum cap.
    Home mechanics wouldn't know about the o-ring and they would lose it
    when they disassembled the carbs for cleaning. The o-ring sealed a
    passage in the pilot air circuit.
    Two Wheels Good, Four Wheels Bad, Jul 14, 2007
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  3. Greg

    Greg Guest

    Are they Mikuni CV carbs? I never heard of a Mikuni CV carb that had a
    Yes. Mikuni BDST 38-B67
    Impossible. I could send photos if you wanted to see better what I'm
    trying to explain
    I'll check the above mentioned.
    The diaphragm actually shrunk.

    I'll try a little vaseline.
    I know the o-ring your explaining because I did
    lose it and replaced it with a new one.

    Thanks for your input.

    Greg, Jul 14, 2007

    Italian is OK, I can deal with Italian.

    Semi-downdraft carbs have some extra parts inside the float bowl
    to make them work like side-draft carbs.

    #3 is the main jet and pilot jet holder

    #6 is the main jet

    #8 is the idle jet

    #11 (on the second drawing) is the pilot air jet

    #16 has the float bowl pivot pin and the float valve

    #13 is an air cut off device that shuts off and air passage when the
    throttle is closed. This reduces farting and backfiring as the
    motorbike slows down.

    #17 is the anti-tamper plug that conceals the idle mixture screw.

    Idle circuit cleanliness is absolutely essential with CV carbs.

    Engines equipped with CV carbs start hard and don't run worth a damn
    if the idle ports and passages are all plugged up.

    If you decide to remove the idle mixture screw for thorough cleaning
    of the idle mixture ports, turn the idle mixture screw all the way
    clockwise until it just seats.

    Count the number of full turns and fractions of turns before removing
    the screw, spring, tiny washer and tiny o-ring. Write the number of
    turns down and keep the two screws separate.

    Squirting aerosol carb cleaner (like Berryman B-12) through all the
    ports and passages will solve many problems with CV carbs.

    What you squirt through the idle jet must come out through the single
    idle mixture port that is controlled by the idle mixture screw, and it
    must come out the three acceleration transition ports that are
    controlled by the bottom edge of the throttle butterfly.

    The aerosol cleaner must also come out the pilot air jet in the
    carburetor mouth. In order to get a good flow of carb cleaner out of
    every port in the idle mixture circuit, I will cover two ports with my
    fingers, and squirt aerosol cleaner into one port until it flows
    easily out the remaining port.

    Then you can reinstall the idle mixture screw parts in the correct
    order and turn them all the way in, then back them out according to
    the numbers you wrote down and you should be back to the factory
    settings, unless some amateur mechanic has tweaked with the screws.

    Turning the idle mixture screw counterclockwise richens the idle
    mixture. Idle mixture is very important between fully closed and 1/8th
    throttle. That's when you get your all-important off idle

    Amateur tuners get into trouble by expecting the idle RPM to continue
    to increase as they turn the idle mixture screw counterclockwise. They
    get to the point where the idle mixture is so rich the idle RPM
    actually slows down and they compensate by turning the master idle
    knob up.

    This uncovers the three transition ports under the throttle butterfly
    and the engine idle will hang up at high RPM when they blip the
    throttle. This effect is more pronounced on inline four cylinder
    engines and won't be as noticeable on a V-twin.

    However, too rich is still too rich. It wastes fuel and soots the
    spark plugs.

    These carbs are set up with really large jets. They must waste a lot
    of fuel keeping the engine cool.The main jets are huge, #140.

    With a huge idle jet (42,5) the idle mixture screw setting specified
    is 3 turns for the horizontal cylinder and 3-1/4th turn for the
    vertical cylinder. The needle clip is in the 4th slot.

    Be sure you have the spring seat and the two washers shown in the
    second drawing assembled in the correct order.

    Some amateur mechanics manage to get both washers on top on the needle
    clip and the needle sits so low the carburetor comes on the main jet
    very late.

    Also, hold the rubber diaphragm up to the light and look for tears and
    holes. The carburetor cannot come on the main jet if the diaphragm

    If you have a torn diaphragm, do not fear the greed of Bologna.

    There is a company in England that makes diaphragms a whole lot
    Two Wheels Good, Four Wheels Bad, Jul 14, 2007
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