Modifying carbs, will I need larger jets?

Discussion in 'Motorbike Technical Discussion' started by Masospaghetti, Jun 26, 2006.

  1. Hey all --

    I have a 1984 KZ700, 9000 miles. Right now it's pretty much all stock
    except for a k&n filter in the airbox. I have a needle and jet kit that
    I am planning to install along with k&n pod filters. My question is,
    since I am already changing the jets with the kit I bought, will I need
    to get even LARGER jets because of the pod filters?

    Masospaghetti, Jun 26, 2006
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  2. Masospaghetti

    FB Guest

    Nope. If you bought one of those silly Dynojet "stage III" kits, the
    jets that they give you are plenty large. They would be large enough
    for high speed riding on a race track.

    The problem with Dynojet's home made jets is that they aren't all that
    precision made, and the numbers don't correspond with Mikuni and Keihin

    Your KZ700 probably came with about #100 main jets and your engine
    probably doesn't need anything bigger than a #115 or #120 main jet, but
    how can you even tell what the Dynojet numbers mean?

    They have some tiny little being with a hammer and a set of number
    punches marking each aluminum jet with their own secret number, but
    Keihin and Mikuni make precision *brass jets* and we KNOW what the
    numbers on the jets mean.

    The jet numbering scheme of Mikuni and Keihin jets is based upon a #
    100 mainjet having a precision hole that is 1.000 millimeters in
    diameter. A # 120 main jet has a hole that is 1.200 milimeters in

    The # 120 jet doesn't pass 20% more fuel, since the area of the hole is
    pi time the radius squared. If you do the math, you will see that the
    #120 jet has 44% more orifice area.

    It's very hard for me to imagine that your KZ700 is going to need 44%
    more fuel just because you switched to Pods. You would have to build a
    radical high compression, high RPM engine with radical camshafts to
    ever come close to needing that much extra fuel.
    FB, Jun 26, 2006
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  3. Thanks for the info.

    The kit I actually bought is a K&N jet and needle kit, would this be the
    same as the Dynojet kit? And given what you've said, should I buy some
    Mikuni jets as well?
    Masospaghetti, Jun 26, 2006
  4. Masospaghetti

    FB Guest

    Are the jets in your kit made of aluminum or brass? Dynojet makes their
    home made jets out of aluminum. Keihin and Mikuni make their precision
    jets out of brass. If you have a Kawasaki, it probably has Keihin
    carbs, so you would buy Keihin jets.

    The difference between Keihin and Mikuni is that Keihin doesn't make a
    "half sized"
    jet. Both Mikuni and Kehin make a #100 jet, but the next Mikuni size
    would be a #102.5 and the Keihin size would be #102.

    The difference in hole size would be about 2/10,000ths of an inch...

    Go ahead and try the kit to see what it does. You get the effect of
    bigger mainjets at full throttle, and if the vacuum is too low to pull
    the needles all the way out of the needle jets, you never get to use
    the main jet at all.

    If you can get the engine warmed up and find a long open stretch of
    road where you can run full throttle for two or three miles, you can
    get a good feel for whether the main jet is too large or too small by
    rolling the last 1/8th of a turn on and off. If the
    engine howls at 7/8th's throttle, but you start hearing invidual bang
    bang banging of the cylinders, the main jet is too large, you need
    smaller mains.

    OTOH, if the engine howls at full throttle, you might be jetted just
    right. That's when you want to do a "plug chop". You run the engine at
    the highest speed the motorcycle will go for a few miles with new spark
    plugs, then you push the kill button and pull in the clutch and coast
    to a stop and pull out the spark plugs and "read" them.

    You're lookign for a narrow black ring around the center insulator,
    deep inside the spark plug. The ring might be 1/8th of an inch wide. If
    there is more soot than that, the engine is jetted too rich.

    OTOH, if there is no soot on the insulator, or you see little black
    specks that look like pepper burned onto the spark plug, that indicates
    the engine is jetted too lean.

    The next possibility is little shiny silver balls of piston material
    melted onto the spark plug. That means that the jets are really too
    small for full throttle use.
    FB, Jun 27, 2006
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