Ping Mr krusty and anyone else that can help! carb help required again...

Discussion in 'Motorbike Technical Discussion' started by messenger1, Aug 29, 2005.

  1. messenger1

    messenger1 Guest

    Greetings! still having carb issues here :( 1982 KZ1000 overbored to 1105
    with a set of 4-2 headers, GSXR Mikuni radial flatslide carbs. Cleaned for
    the 4th time, kremed the tank this time, installed 2 high flow fuel filters
    inline as well as clear fuel line (so I can visually inspect for adequate
    flow) Starts great, idles crappy, popping through 1 or maybe 2 of the carbs,
    when riding it it has lots of power with partial (about 1/4) throttle but
    bogs and spits horribly when trying to rev past 5000rpm or semi to WOT.
    tried running with the choke on, it smoothes right out at about 3000rpm, but
    still pops and sputters above that with semi or WOT. Valve clearances are
    all within specs, lots of compression, ignition seems ok....I'm getting
    nowhere, any more ideas?? btw, might have goofed when I set the pilots at
    3/4 turns out hoping to rectify this, what would the symptoms of too many or
    not enough turns out on these be??

    any advice/help appreciated!
    messenger1, Aug 29, 2005
    1. Advertisements

  2. My advice is this. Get it to idle smooth and work from there. It is
    difficult if not impossible to get a good running engine if it won't idle
    right. Did you try spraying a water mist on the exhaust pipes while the
    engine is at an idle? Are all the pipes getting equally hot?

    Have you seen this diagram?

    R. Pierce Butler, Aug 29, 2005
    1. Advertisements

  3. Those headers could be letting that engine breathe easier than before. Its
    possible now that the fuel mixture is a little leaner than it should be.
    That overbore will contribute to that issue as well. You might want to
    consider raising your carb needles up a notch. If that makes an improvement
    with your mid-range then try going richer with your main jets. Let us know
    how you make out :]
    Jeff Rutledge, Aug 29, 2005
  4. Those are the flatslides that supposedly fit a "GSXR" and you got them
    from a guy on eBay? What you need to do is go to and
    click on the Online Catalog link. Scroll down to Mikuni Carburetors and
    download the RS carburetor .pdf file, the one on jet needles, and
    anything you can find on needle jets, idle jets, and general carburetor
    tuning. Hopefully, you will be able to figure out *exactly* which RS
    carbs you have. I don't remember if the needle jet has P-4 or Y-6
    stamped right on it or not. You'd need a set of precision metric pin
    gauges to identify what needle jets you have by sticking a pin down the

    The inside diameter of the needle jet is very important as you open the
    throttle from 1/8th to 1/4th throttle. Then the diameter remains
    important until you open the throttle from 1/2 to 3/4th's throttle.
    That's when the jet needle taper becomes more important. From 3/4th's
    to full throttle, you're running on the main jets.

    But amateur shade tree tuners don't realize that the main jets are not
    the main tunable element in their carbs. They start throwing brass at
    their carbs and brass costs $$$. Most of the brass I have was given to
    me by shade tree tuners who found no further use for it after jetting
    their carbs for the only race track around here...

    There were four RS flatslide kits with the correct 77mm-93mm-77mm carb
    spacing to fit Suzuki and Kawasaki inline fours. The RS34's would be
    too small for your machine, but they would fit a KZ-750 with
    aftermarket exhaust and K&N filters.

    The RS36's would work on a KZ-1000 with aftermarket exhaust and K&N's.

    The RS38's would be for a KZ1000 with aftermarket exhaust, K&N's,
    overbore, valves and head work, increased compression, higher RPM, or a
    complete race engine.

    The RS40's would fit a KZ1000 full race engine.

    The part numbers may be acid etched on the side of one of the outer
    carburetors, or there might be a metal tag attached to a screw on the

    The main jet size may have been changed by a previous tinkerer, but all
    of those carbs came with 17.5 pilot jets and the pilot screw was set
    1/4 to 1/2 a turn open. That's a nominal setting, if a tinkerer has
    installed larger pilot jets thinking he's going to get better
    acceleration that way instead of adjusting the accelerator pump, the
    pilot screw might need less than 1/4 of a turn open and the exhaust
    sound might be "toot-toot, toot-toot, toot-toot" at idle, indicating
    the idle mixture is far too rich. Black smoke might come out the
    exhaust as the rider rolled on the throttle, then the engine would
    clear out at wider throttle openings as more air blew the excessively
    rich idle mixture out.

    The RS34 and RS36 carbs came with P-4 needle jets. That's the brass
    tube that the main jet screws into. The inside diameter of a P-4 needle
    jet is 2.670 millimeters, and the jet needle attached to the slide goes
    down into that needle
    jet and provides a *variable orifice* with MORE restriction to fuel
    flow than the main jet. In other words, the difference in area between
    the needle jet hole and the jet needle is smaller. In other words,
    gasoline is sucked through a smaller *hole* than the main jet as you
    just open the throttle a little.

    So, from fully closed throttle to 1/8th throttle, you're running on the
    idle jet, the fuel air mixture that gies around the throttle slide and
    comes through the idle bypass passages and that tiny bit of extra gas
    that comes past the pilot mixture screws.

    IOW, all the idle mixture doesn't come past the pilot screw, only about
    1/4 of the needed fuel comes past the pilot screw to fine tune the
    mixture. When a shade tree tuner starts screwing around with idle jets
    and he doesn't understand that subtle point, he's going to have an
    engine that gooes "toot-toot" at idle, and the RPM hangs up when he
    blips the throttle because the idle jet is too big and the pilot screws
    are open too far and he's compensated by adjusting the idle RPM higher,
    or he's going to have an engine that spits back through the carbs,
    pings on acceleration, runs too hot when warmed up, but takes forever
    to warm up and is hard to start on a cold morning.

    Now, back to jet needle and needle jets and you've just opened the
    slides further than 1/4 throttle. The fuel air mixture is partly coming
    from the idle jets and the pilot mixture, however right-sized or
    wrong-sized the jets may be, and however screwed up the shade tree
    tuner may have adjusted the pilot mixture screw. But, theoretically,
    from 1/4 throttle to 1/2 throttle, the fuel air mixture is supposed to
    be MORE influenced by the diameter of the needle jet. Again, that's the
    brass tube that the tapered needle goes in and out of. The RS34 and
    RS36 carbs had the P-4 needle jets stock, with their 2.670 millimeter
    holes. They came with a 9DZH1 needle that was 2.500 millimeters in
    diameter and the nominal jet size for a KZ1000 with an aftermarket
    exhaust and K&N's was a #120 round jet.

    But the RS38 and RS40 carbs had MUCH larger needle jets, they had Y-6
    needle jets. And the hole in a Y-6 needle jet was 3.130 millimeters in
    diameter, so it needed a fatter needle so the jet needle/needle jet
    worked as a variable orifice properly. The fatter 9CHY3 needle that
    came in those carbs was 2.98 millimeters in diameter, and you can work
    out the areas of the hole in the needle jet and the area of the jet
    needle and whatever mainjet you have in there and see if the variable
    orifice is actually smaller than the area of the main jet or if you're
    sucking excess gasoline like a toilet flushing, or what...

    The nominal main jet in an RS38 in a KZ1000 engine up to full race
    modification was a #135 round jet.

    Clear fuel line will make you have doubts about whether you're getting
    enough fuel flow when you see big air bubbles in the lines, bleed the
    air out, and then see big bubbles again...
    As I said above, the pilot mixture screw only adjusts a small
    percentage of the idle fuel air mixture. The correct number of turns in
    or out is going to vary if the previous owner has changed the idle jets
    from the as-delivered 17.5 idle jet.

    Too big an idle jet/too many turns out makes the exhaust go
    "toot-toot", you get black smoke, the idle RPM hangs up when you blip
    the throttle. When cold, the idle RPM is far too high, so you have to
    fiddle with the choke to keep the engine RPM from racing. If you adjust
    the idle mixture so it seems OK while the engine is just warm enough to
    idle smoothly, you find that the idle RPM is much too high when the
    engine is hot. It drives you crazy, you get drunk and kick the dog.

    Too small an idle jet/too few turns out results in an engine that is
    hard to start, warms up slowly, spits out the carburetor, pops out the
    exhaust, stalls when you roll off the throttle.
    krusty kritter, Aug 29, 2005
  5. messenger1

    messenger1 Guest

    Thanks to all that have replied! I have had to take a crash course in carbs
    here, I deserved it for getting into this ;/ I have since last posting,
    raised the jet needle to its highest position, now I can get it running and
    accelerate half decently. Still spitting and sputtering from 1/2 to WOT
    which based on my newfound knowledge (please correct me if I'm wrong!)
    indicates I need to replace at the very least the jet needle and the needle
    jet. I'm not sure about whether the main has to be resized as I'm assuming
    I should get rid of this mid to wot issue first?? BTW does anyone have a
    special method for removing the jet needle in these RS carbs? What a pita, I
    undo the 2 allen screws, ratchet the throttle shaft back, and use a magnet
    to lift it, then tweezers to snake it past the lever and plate assembly.

    again thanks all for the info and shared knowledge!
    Tom B
    messenger1, Sep 4, 2005
  6. Did you download those .pdf files from Sudco to determine which carbs
    you have and inspect them to see which main jets, needle jets, needles,
    and idle jets you have?

    Do the accelerator pumps work? The accelerator pump rods are supposed
    to be disconnected before re-jetting the carbs to avoid upsetting the
    test with excess fuel being squirted into the venturi.

    The normal order of tuning is to start by sizing the main jet with wide
    open throttle runs and (with slide valve carbs)rolling the throttle on
    and off from full throttle to about 7/8th's throttle and listen for the
    exhaust sound to change. If the exhaust starts thudding and you hear
    individual bangs instead of the desired howling sound, the main jet is
    too large. You want to size the main jet so the engine achieves the
    highest RPM possible on the longest straight on your test course.

    After you've completed the main jet selection, you can work on the
    needle position. You normally should NOT have to raise the needles all
    the way. That tends to indicate a main jet that's too small

    As I said previously, there were two size needle jets that came stock
    in those carbs. The Sudco catalog doesn't give all the dimensions for
    the taper of the needles used in the RS carbs the way it does for
    needles used in earlier carburetors. But, you can chart the needles
    yourself if you measure them every 10 millimeters, starting them from
    the top. Got a vernier caliper?

    Earlier needles were charted in the catalog according to their diameter
    measured every 10 millimeters starting at the top end of the needle.
    The highest e-clip slot on the needle would be about 3 mm from the top,
    the lowest slot would be about 7 mm from the top of the needle.

    You can figure out which needle jets and jet needles you have if you
    measure the upper diameter of the needle *before* it starts tapering to
    a smaller diameter. If you calculate the area of the hole in the needle
    jet and calculate the area of the jet needle where it is NOT tapered
    and subtract the area of the jet needle from the needle jet in the
    untapered area, you'll see that the difference in area is very small.

    But, if you measure the jet needle's diameter at a point about 3/4's of
    the way down from the top, the area of the needle jet minus the jet
    needle's area should be about the same as the area of the main jet.
    That's when the main jet takes over, it then controls the flow of fuel
    through the needle jet jet needle orifice.

    So, from 1/4 to 1/2 throttle, the needle jet diameter has the greatest
    influence on mixture, the jet needle's taper takes over from half to
    3/4's throttle, and the main jet controls fuel flow from 3/4's to full

    Once you get the main jet sized correctly, you can adjust the needle
    position, then prime the accelerator pumps and hook up the pump rods
    and adjust the stroke according to the Sudco .pdf file...
    krusty kritter, Sep 4, 2005
  7. messenger1

    Ron Seiden Guest

    Well, that surge of info just cured me of ever wanting to mess with the
    carbs on my bike. I've rebuilt auto carbs (including a Quadrajet done on a
    picnic table) successfully, but fine tuning a set of 4 bike carbs seems like
    just too much trouble. I guess this winter I work on that single carb
    conversion project...
    Ron Seiden, Sep 10, 2005
  8. Whether it's a multicarb set up, or a single pot on a log manifold, the
    carburetor circuit tuning sequence is the same, so you're not going to
    escape the tuning problems everybody goes through.

    The OP seems to have some very refined racing carburetors with radial
    flat slides and accelerator pumps, but he bought them on eBay and
    doesn't seem to know exactly what bore the carbs are, so I've been
    shotgun troubleshooting with him to help him figure it out...
    krusty kritter, Sep 10, 2005
  9. messenger1

    messenger1 Guest

    and I'm very grateful, for the help! Here's the most recent update. I've
    swapped the mains for a larger set, don't know the sizes, everyone that was
    around the house here while I had the carbs disassembled (young eyes old
    eyes drunk eyes and sober eyes) took a stab at looking at the mains with a
    magnifying glass but no one could decipher the markings as anything that
    would identify the size. So I pirated the visually larger mains from an old
    set of CV carbs from one of my old KZ's. I installed these, set the needle
    at the #3 position as a starting point and re-installed them. Voila. 100%
    better. Now I only have a very bad stutter at extreme WOT. However, still
    spits back through the carbs, smokes at startup and low idle and stalls
    intermittently till warmed up.
    This pretty well describes where I'm at! I've set the pilots at about 1/2
    turn out. Still have too crank up the idle knob to get it to idle with no
    choke, then turn it down once it gets warm to prevent it from racing. It
    does not however, hang-up at higher rpms. Smokes like the rings are shot
    till I rev it too clear out the passages, then once its warmed up behaves
    like it should except at wot.

    thanks again

    Tom B
    messenger1, Sep 11, 2005
  10. Extreme WOT is an oxymoron. You've either opened your throttle all the
    way or you haven't.

    The diference between how the engine runs at 7/8th's throttle and how
    it runs at WOT is a major CLUE about whether your main jets are too big
    or too small.

    It marks the final transition from mixture being controlled by the
    needle and mixture controlled by the main jet.

    If the engine sound is a high pitched howl at WOT, you're either jetted
    about right, or you're lean. If lean, the engine won't pull as hard as
    it should and it will get hot. If the exhaust note changes to where you
    hear distinct thuds, the mixture's a little rich. If it stutters and
    blows black smoke, you're too rich, and when you roll the throttle back
    to 3/4th's to 7/8th's throttle, the needle jet area minus jet needle
    area is once again smaller than the area of the main jet, so the nj-jn
    area is controlling fuel flow to the venturi. The engine goes back to
    its normal howl, but not as loud at it was at WOT.

    Surging at WOT or 3/4th's to 7/8th's throttle indicates a very lean

    If you still have an airbox, and you've removed the snorkel, put it
    back in, engines will surge like crazy with the airbox lid on and the
    snorkel removed,
    as the resonant frequency of the airbox is changed and air does NOT
    like to flow smoothly through a simple hole in a flat surface, it gets
    seriously turbulent.

    During surging, the engine will not hold a steady RPM, the motorbike
    bucks like an angry horse that doesn't want to be ridden. Surging is
    particular annoying on the racetrack in high speed sweepers where
    you're leaned over at a severe angle. You have enough problems with
    traction and cross winds, a surging motor makes for a wild ride.

    You might mistake surging for stuttering by the sound, but when you do
    your plug chop there won't be much color on the insulator nose.

    So, what does your plug chop look like? I've described how to do a WOT
    plug chop many times, you can google for "[email protected] +plug
    chop" to read it. Is the insulator nose all covered with black soot, or
    is there just a little bit of greyish-white at the very tip and the
    rest is fluffy black?

    As I've said many times, you're looking for a black sooty ring maybe an
    1/8th of an inch wide way deep down inside the spark plug, where the
    insulator and the steel shell meet. The rest of the insulator nose
    should be white, but not rough looking. That narrow ring indicates a
    safe mixture at WOT.

    Two-valve engines take longer to burn their mixture and four valve
    engines, a 2-valver takes about 90 degrees of crankshaft rotation to
    burn the mixture, a modern 4-valver takes about 64 degrees of
    crankshaft rotation to burn the fuel, so there's less fuel to deposit
    on the interior surfaces of the combustion chamber and spark plug.

    Since they burn their mixture in about 2/3rd the time a 2-valve engine
    completes burning, a 4-valve engine will have a really narrow ring of
    krusty kritter, Sep 11, 2005
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.