84 KZ700 bogs down at hgh RPMs

Discussion in 'Motorbike Technical Discussion' started by Masospaghetti, Aug 1, 2006.

  1. Hey all --

    I know this has probably been posted a hundred times but I couldn't find
    a guide on google. The bike is a KZ700, 1984, 9900 miles on the clock.

    I just installed K&N pod filters and a stage 1 jet kit which included
    new needles and main jets. The bike runs well up to about 6000 RPM, then
    it starts to lose power and bog down.

    Would this be the main jet? Would the addition of the pod filters
    require a larger main? Note that the jet needles are one slot from being
    as rich as possible.

    Masospaghetti, Aug 1, 2006
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  2. Masospaghetti

    ade Guest

    Hi James

    does the bike bog down when you have the throtle open fully after
    6000rpm? Try to ease the throttle just before 6000rpm and check if the
    machine is still accelerationg slowly. If that is the case the problem
    is in carburators. I had a similar problem before, when i changed my
    jet kit, it sounds like a too rich mixture. Your have to calibrate the
    carburators to match the new jets. The best you can do is to google out
    a user manual and i'm sure this procedure is well described there.

    check here:

    Best of luck
    ade, Aug 1, 2006
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  3. Thanks for the response.

    I will try that -- however, if it IS too rich, that would mean too large
    of a main jet? Or would it be the needle clip position?
    Masospaghetti, Aug 1, 2006
  4. Masospaghetti

    FB Guest

    Yes, and I have explained to you how your carburetors work at least a
    dozen times.

    It doesn't matter what kind of motorcycle you have, carburetor tuning
    principles are the pretty much the same for all Japanese motorcycles.
    You have to understand how the carburetor works in order to tune it.
    Put the stock needles back into the carburetors. Install original
    equipment Keihin or Mikuni mainjets. Put the stock airbox and filters
    back on.

    Throw the Dynojet kit away.
    Yes, it's probably drowning in excess gasoline because Dynojet jets are
    for the race track and for running at wide open throttle, and street
    riders usually don't do that.

    Newbie riders talk to their buddies about motorcycles and their buddies
    tell them that they installed a Dynojet kit and they brag about how
    much better the engine runs now. The proud owners of the Dynojet kits
    will never admit that they couldn't sort out the problems of
    excessively rich running and overly fast idling and blubbering at full

    But the newbie goes ahead and buys the Dynojet kit to keep up with his
    buddies and he inherits the same tuning problems.
    Yes, but no more than about 4 sizes larger than stock.

    If your engine came with #100 main jets, a #110 mainjet is four sizes
    larger. The sizing goes #100, #102.5, #105, #107.5, and #110 for Mikuni
    jets and Keihin jet sizes would be almost the same but Keihin doesn't
    use the half sizes.

    It is impossible to say what Dynojet's numbers on the side of their
    home made jets mean. Throw them away.
    Put the stock needles back into the carburetors and put the clip in the
    second slot from the top.

    Remember that the needles are going up and down in the needle jet,
    which is the brass tube that the main jet screws into. The needle and
    needle jet, in combination, create a VARIABLE ORIFICE.

    The main jet is a FIXED ORIFICE. The size of the hole doesn't change
    during operation.

    If you don't know what an orifice is, it's a CALIBRATED HOLE that air
    or fluid passes through in a controlled manner.

    The main jet and the needle jet/jet needle combination are TWO ORIFICES
    IN SERIES. The first orifice is VARIABLE, the second orifice is FIXED.

    The carburetor sucks more and more gasoline through the main jet as the
    throttle slides lift from the closed position. But the needle and
    needle jet keep the maximum amount of gasoline that the main jet could
    pass from reaching the carburetor throat.

    Once the needle lifts so far out of the needle jet, that VARIABLE
    ORIFICE area BECOMES LARGER than the FIXED ORIFICE area of main jet.

    This allows the maximum amount of gasoline that the main jet can pass
    to get to the carburetor throat where it meets the stream of air
    passing into the engine.

    All the needle clip position does is control the point at which the
    above transition from variable to fixed orifices takes place.

    But shade tree mechanics do not seem to understand this subtle point.
    They read the tuning guides that talk about the various stages of
    tuning and they think that
    the idle jets and main jets and needles all work separately. They seem
    to think that a carburetor is some kind of fuel injection system.
    That's not true. The idle jets, main jets and needles all have to work

    But the shade tree mechanics raise the needle far too high and the
    transition to the main jet takes place too early, when the slides
    haven't lifted far enough to allow sufficient air to pass through the
    carburetors to allow the mixture to burn cleanly.

    So, I recommend that you reinstall the stock parts and forget about the
    Dynojet fiasco.
    FB, Aug 1, 2006
  5. The reason I did any of this to begin with is because the bike had a
    really bad flat spot in the midrange WITH everything being stock, and I
    was tired of taking those airbox hoses off and on every time I needed to
    tweak the carb.

    With the stock needles, the bike would hardly run with the K&N pod
    filters installed.
    The spark plugs are still a nice light tan color, and the bike ran worse
    cold than it does hot.
    If it makes any difference, this is only a "stage 1" kit, and the
    needles dont seem to taper much more (if at all) than the stock ones.
    The bike had 105 main jets stock, the Dynojet main jets are supposed to
    be 110's. I have some 112.5 and 115 jets on the way (not from Dynojet).
    The bike definitely will not run correctly with the stock needles as
    they are. Should I put washers under them and get rid of the dynojet
    needles? The stock needles are not adjustable.
    Masospaghetti, Aug 1, 2006
  6. Masospaghetti

    FB Guest

    What exhaust system do you have, a 4-into-1? You CANNOT tune the
    midrange flat
    spot out by messing around with jetting, but Dynojet still sells their
    bogus "Stage I"
    kits to riders that don't know that fact.

    When I complained to the Dynojet technical representative about being
    unable to use third gear in slow corners because of the flat spot, and
    being unable to use second gear in the same corner because of lean
    surging, he told me that I should rev up the engine and just "shoot
    through the flat spot" on acceleration.

    That would work fine on a long road racing course, or at the drag
    strip, but I was a canyon rider and I needed engine flexibility in the
    slow corners.

    One trick that helps the midrange flat spot is to advance the ignition
    timing by about five degrees by installing an aftermarket ignition
    advancer or by slotting the plate that the ignition pulser coils are
    attached to.

    Advancing the ignition gives you more midrange torque at the expense of
    slightly less top end horsepower. Ignition adbancing works by
    increasing the pressure in the cylinder as it approaches top dead
    center and the combustion process is a little hotter.

    You risk having your engine pre-ignite or even detonate. You can melt a
    hole in your piston or even have the engine seize if you don't know
    what you are doing.

    The first indication of mild pinging is black specks on the nose of the
    spark plug that look like pepper. Worse pinging can be heard, it sounds
    like a loose valve. You will see little shiny silver balls of aluminum
    on the plug nose if you are beginning to melt a hole in a piston.

    I have seen spark plugs filled with melted aluminum from engines that
    melted their pistons and seized.

    So, messing around with ignition timing requires some knowledge of how
    it works, what you are trying to accomplish and what can happen.

    And, ignition advancing will cause the engine to lean surge, at part
    throttle, so you need to mess around with the idle mixture screw again!

    I wish I could accurately describe the groaning sound that comes from
    the exhaust system of an engine with lean mixture, hot spark plugs, and
    a slightly advanced ignition.

    My old GSXR750 is set up like that. But when the engine pulls through
    the midrange flat spot and heads for the rev limiter, the engine sounds
    like a trumpet, the music is sweet.
    Well, maybe what you are encountering at 6000 RPM is lean surging. I
    propose that you reinstall the stock needles, in the correct position,
    and open up the idle mixture screws about one turn and see what
    #115 main jets are probably the largest your 700cc engine could ever
    use, at wide open throttle on a race track.

    Dynojet's home made jets are NOT the same as Mikuni or Keihin jets. The
    Japanese jets are what is called a "reverse venturi" type. If you look
    at the Sudco International website or the Factory Pro website, they
    probably have a cutaway view of a reverse venturi jet which has,
    obviously, a venturi shape with a precision calibrated hole in the

    Home made imprecise Dynojet jets just have straight holes drilled in
    the aluminum.
    Who can say what the flow rate is, compared to a Mikuni or Keihin jet?
    All shimming the stock needles would do is fine tune when the carb
    comes onto the main jet, which is around 80% to 90% throttle.

    I would bet your "bogs down" problem occurs at part throttle.
    FB, Aug 1, 2006
  7. Currently I still have the stock exhaust system installed.
    So with just the K&N pods, the stock needle and jets will work fine? I
    really would like to use the pods if at all possible because working
    with the stock airbox is a nightmare, and the intake roar is awesome.
    Yeah, it occurs at all throttle positions, actually -- just at the
    higher RPM range.

    I appreciate your input.
    Masospaghetti, Aug 2, 2006
  8. Masospaghetti

    FB Guest

    I was never able to get my GSXR to run right with the stock exhaust and
    the Dynojet kit with K&N separate filters. It didn't "wake up" and
    scream until I installed a LOUD
    Ypshimura competition only pipe.

    Yes, the engine now has 15 to 20 more horsepower than it had when the
    airbox and stock exhaust was installed.

    But, I don't dare ride the machine around my neighborhood. It's a
    racetrack refugee that can only be uncorked in the open countryside.
    An internal combustion engine is an AIR PUMP that sustains its pumping
    action by burning a fuel/air mixture inside of it. This air pumping
    action consumes one third of the power produced by burning the fuel.

    The amateur tuner becomes an amateur tuner when he sees what other,
    more successful tuners have done. But he often doesn't understand why
    he is doing the things he is trying. He is emulating others. He is
    monkey see, monkey doing...

    The idea of horsepower adding modifications is to get the engine to
    pump more air in, by the mechanical motions of the piston, or by using
    acoustic wave motions that take place in the exhaust system.

    The engineers worked to design a complete system of airbox, camshaft
    timing and exhaust system to get the engine to work moderately well
    through a certain
    limited RPM range.

    Engineers accept some compromises in engine performance to get just
    enough extra power to beat their nearest competitor in the fierce
    contests at the $tealer$hip$.

    But mostly the engineers work to get a nice wide midrange performance
    band because their customers mostly ride on the street.

    The public wants to ride the most powerful machine in the class, but
    they don't want to pay an arm and a leg for a limited production race
    replica. They want the look and the image so they can feel like they
    are keeping up with their riding buddies.

    And, nobody really wants a quirky-running motorcycle. They want
    convenience and
    reliability. But the never-ending competition of egos between riding
    buddies eventually leads to a newbie amateur tuner trying to install a
    Dynojet kit.

    And, they wind up with a quirky running engine and they don't
    understand it.

    You removed the air box and lost whatever power enhancing effects that
    it offered the engine. You put separate filters on, and they would
    normally allow more air to flow into the engine.

    But you left the stock exhaust system on, and that system plugs up the
    flow of burnt gasses out of the engine. So the engine has to work
    harder to pump out any extra air that you managed to get into the

    Imagine the proverbial mean alligator with a huge mouth that can eat
    everything in sight. Problem is, the alligator is constipated and
    that's what makes him so mean.

    No matter how much the constipated alligator wants to eat, he cannot
    ingest more food until he gets the last meal out.

    With the air box removed, there is no opposition to the adverse
    pressure waves coming back from the exhaust system. And the newbie
    rider uses more throttle, trying to test out his wonderful new Dynojet
    kit that he has so much hope for.

    And HEAT builds up in the restricted exhaust system, and it forces
    exhaust gasses back to the cylinder and the exhaust gasses force air
    backwards through the carburetor, out to the air filters, before the
    pressure wave reverses and flows back through the carburetor.

    The engine staggers between lean surging and puffing soot out the

    A carburetor is a STUPID device, it doesn't know which way the air is
    flowing through it, and it sucks gasoline out of the float bowl on BOTH
    passes of air.

    This drives the newbie tuner crazy. What the heck is happening, when
    NOTHING he does to the carburetor cures his strange, self-inflicted

    The carburetor tuner's job is to tweak the carburetors, once the engine
    is able to pump more air.

    He uses his extensive knowledge of carburetors and jetting and what
    each circuit of the carburetor does and how those different circuits
    OVERLAP each other.

    The Dynojet scam takes advantage of the amateur tuner's lack of
    knowledge, and millions upon millions of those bogus jet kits have been
    sold to people who never got their engine to run right after installing
    the kit.

    And, the newbie tuner will never admit that he was fooled by the
    Dynojet scam, he has invested $90 to $150 in the project, so he has to
    claim that it works really well.

    My recommendation to you is to restore the carburetors to stock and
    reinstall the air box. Lubricate the rubber boots so they will slide
    over the carburetor inlets easily.

    Limit your carburetor tuning to tweaking the idle mixture screw. If the
    spark plugs tell you that the engine is running too lean but the
    carburetors are spiffy clean inside, it might be a problem with the
    restrictive stock exhaust system.

    If you decide to install a freer flowing exhaust system, your Dynojet
    kit will work better with the K&N's. But your neighbors will hate you.
    FB, Aug 2, 2006
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