Carb issues (again)

Discussion in 'Motorbike Technical Discussion' started by messenger1, Apr 2, 2006.

  1. messenger1

    messenger1 Guest

    Hi all. Once again I need to tap into the collective wisdom of this forum.

    I have installed a stage III dynojet kit on my 1985 GPz900( K&N's and a
    Muzzy 4-1). Reason for this installation? Last year I had some starting
    issues (turned out to be valve adjustment duh on me) and while cleaning and
    recleaning the carbs, I thought a jet kit and K&N's would be easier on me
    than trying to get those damn little carb to stock airbox rubbers back on.

    Through trial and error, I have tweaked the clip settings on the jet needle
    (its now in the 3rd from top groove) and the bike runs phenomenally well
    right up to WOT once it "clears its throat" after about 1/4 to 1/3
    throttle. The problem I'm having is from about 1/16th to 1/4 throttle, it
    sounds like a 2-stroke and spits through the carbs. Its idles fine, although
    again, it intermittently spits through one or more of the carbs while
    idling. I have tried different settings on the air/pilot screw, in
    increments from 1 turn out to 3 turns out with no noticeable change.

    Any ideas/help/input greatly appreciated!

    Tom B
    messenger1, Apr 2, 2006
  2. messenger1

    FB Guest

    No, you didn't buy an easy *fix* for a problem when you bought a
    Dynojet kit, you bought a bucket of worms to sort out!

    So what if Dynojet "optimizes" main jet size on their own Dynojet
    dynamometer? They make their own friggin' main jets and the jets are a
    completely different design from what Keihin and Mikuni make. And, the
    Dynojet numbering system does not
    directly equate with the original Keihin or Mikuni sizing, so you have
    no idea which way to go.

    If you follow Dynojet's vague instruction sheet that comes in the kit,
    they tell you a bunch of tests to do and if the engine fails the test,
    do this or that.

    But Dynojet never tells the purchaser that their Stage III kit is for
    somebody who actually *races* the motorcycle on a real race track, like
    maybe Daytona or Willow Springs, where the rider holds the throttle
    wide open for minutes at a time.

    I advise home tuners to go to the Factory Pro webpages and read their
    CV carburetor tuning guide. Marc Salvisberg gives away information that
    Dynojet keeps secret.

    The main jet sizing strategy according to real racers is to find the
    main jet that "pulls hardest" at wide open throttle when the engine is
    up to operating temperature.

    But, in truth, the correct main jet size is the one that keeps the
    *piston from melting* and the valves from burning during a race of the
    length that the rider really runs.

    Constant vacuum carburetors rarely run on the main jet. You could even
    probably get away with running the stock main jet or one that's just
    maybe two sizes larger than the original jets.

    But, I'll bet you just stuck those stage III racing main jets in your
    carbs, thinking you were going to get a bolt on fix.

    Sorry about that. Sorting out carburetors is a bucket of worms if you
    don't know what you're doing.
    It will probably run better if you get some smaller main jets in there
    and put the needle clip back in the original slot and get the idle
    mixture screws adjusted right.
    Do you mean that the engine sounds like it's gargling gasoline? That's
    called "eight-stroking". Each cylinder fires fires every other
    compression stroke instead of firing every compression stroke
    Does the idle RPM hang up at high RPM when you blip the throttle? That
    can indicate excessively rich idle mixture.

    Spitting back through the carbs occurs when the waste spark fires while
    both intake and exhaust valves are open. There shouldn't even be a
    burnable mixture in the cylinder if the carbs are adjusted correctly.

    I suspect that you may need to adjust the idle mixture screws to only
    1/4 to 1/2 a turn
    if you have fairly large idle jets of around #35 to #40.

    You do realize that the #35 main jet used in a constant vacuum carb has
    an orifice that has four times the area that a #17 pilot jet used in a
    slide valve carb has, don't you?

    A CV carburetor is running mostly on the idle jet until the vacuum
    slide raise high enough to pull the jet needle far enough out of the
    needle jet that the resulting area is larger than the area of the main
    FB, Apr 3, 2006

  3. Not once in all my years of bending wrenches have I thought, "It would be
    *so* much easier to remove the old airbox, install K&N filter that require
    constant maintenance, take apart carbs, put jets in them, play with the
    mixtures for hours or days, possibly burn the valves in the process and
    think that would be easier than installing new airbox rubbers. Nope. Not
    once did I ever think that and I don't know of one person who has until

    R. Pierce Butler, Apr 3, 2006
  4. messenger1

    messenger1 Guest

    Yes, they have their own numbering system, I used the largest sized main of
    the 3 provided with the kit as per their instructions. I will try dropping
    down one size.

    Very vague instructions, none of which pertain to my current sympyoms.
    I will google that up!

    Not really, I knew there would be "some" fiddling and adjustment" required,
    just didn't think it would be this much!
    Amen to that!
    My next per my first reply..
    Yes, thats what it sounds like allright!
    No it doesen't hangup, but I did a a plug chop and it indicates a very rich
    I have NO idea on how to detrmine the idle jet size, there are no markings
    on any of the 4??
    I do now!

    thanks FB

    Tom B
    messenger1, Apr 3, 2006
  5. messenger1

    messenger1 Guest

    In my defense, let me clarify my original post...I bought the bike with
    stock exhaust and airbox. I modified a Muzzy ZX10 4-1 to fit it and
    subsequently developed the usual miss at midrange rpms. While trying to get
    rid of that "stutter" through trial and error adjustment and jetting, I took
    the carbs off several times. I then developed a hard starting problem (which
    turned out to be a totally unrelated valve adjustment issue) and in the
    course of removing and reinstalling the carbs multiple times I had that
    brain cramp regarding it being "easier" to go with the K&N's. I was not
    oblivious to the fact that there would be some tweaking required, but I've
    never owned a bike with a more hatefully tight clearance between the carbs
    and the airbox as this one.

    Tom B
    messenger1, Apr 3, 2006
  6. messenger1

    FB Guest

    Well that indicates that you need to understand how the pilot mixture
    screw works with the pilot mixture system. The idle gas comes in
    through the idle jet, the air comes in through the pilot air jet and
    gas and air are mixed in the passages. Then the passage branches. The
    idle fuel air mixture comes out through three transition ports just
    downstream of the throttle butterfly. The transition ports are NOT NOT
    regulated by the pilot mixture screw, they are regulated by the

    Can I emphasize that any more?

    The pilot mixture screw is only a trimmer screw on the idle mixture
    circuit. it has VERY LITTLE effect on idle mixture tuning, except in
    the mind of the inexperienced tuner, who will think that he needs to
    open up the idle mixture screw a whole bunch and that the idle RPM will
    speed up as he opens the screw.

    It just doesn't work that way. The engine is already getting enough
    idle mixture through the fixed port that's uncovered by the butterfly.

    Then the ignorant home tuner keeps opening the idle mixture screw more
    and more and more and is confused by the fact that the idle RPM
    actually SLOWS down.

    So he compounds his mistake by adjusting the master idle knob to speed
    up the
    idle RPM and this uncovers more of the transition ports and the idle
    mixture becomes ridiculously rich and the engine stutters.

    The normal procedure of tuning the idle mixture circuit is to turn the
    pilot mixture screw IN and turn the master idle knob down as the idle
    RPM speeds up from leaning out the idle mixture.

    It used to be SO EASY on car carburetors. The idle mixture screws were
    right there in the front of the carb. We turned each idle mixture screw
    in until the engine ran rough, then turned it back out 1/4 of a turn
    and we were done, it was time for a beer.

    Motorcycle CV carbs often have such huge idle jets, you might find a
    carburetor whose idle mixture screws was set only 1/8th of a turn open
    from the factory.

    OTOH, one guy told me that his 1200cc Kawasaki UJM replica needed its
    idle mixture screws set about three full turns out. It seems that
    Kawasaki put #35 ilde jets in the carbs for that machine. I've seen
    carbs with #47.5 idle jets and you could hardly open the idle mixture
    screws at all.

    The only other logical possibility is that you've set your floats too
    high. The tolerance is plus/minus 1 millimeter. Setting the floats too
    high (which is too LOW when you have the carbs turned upside down on
    the bench) will make the idle mixture too rich.

    Mark Salvisberg describes the condition as being "soggy".
    Well, if you cannot read the tiny 2-digit numbers on the jets, the only
    practical way to know what you have is to remove the old ones and buy
    four new jets directly from Kawasaki. If you go to
    and look up your machine, you can find out what idle jets are available
    for it.

    Don't even try to remove an idle jet if you don't have a slot
    screwdriver that fits the jet exactly, as you could booger the jet and
    have to drill it out.
    FB, Apr 3, 2006
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