Questions about Mikuni Carbs on a late 80s 600 Katana.

Discussion in 'Motorbike Technical Discussion' started by Wesman, Jun 6, 2005.

  1. Wesman

    Wesman Guest

    Hi all, I got a few question about Mikuni Carbs on a late 80s 600 Katana.
    And I figured this is one of the best places to post my questions. My hope
    is there are a few people out there who could share with me real working
    knowledge for this carbs.

    Here is my story, I picked up a semi running Katana that had been sitting
    on some guys porch. The bike barely ran, he offered to sell it to me. His
    story was that the bike started to run rough, so he took apart the carbs to
    rebuild them, and then it ran worse. He then picked up another set of carbs
    off another bike, which ran better but still had problems. So he rebuilt
    them and then once again, the 2nd set of carbs made the bike worse. I went
    ahead and took a chance and bought the bike and the extra set of carbs.

    I took the 2 sets of carbs apart and mixed and matched the best looking
    parts to build the best single set that I can.

    This is what I have observed, after several tear downs, reassemblies, and

    1) The bike idles good. Pretty clean response from idle to about 2000 rpm.
    2) If the CV slides lift 100% open at wide open throttle the bike has
    excellent acceleration. No missing or stumble for the engine.
    3) The slides have a real hard time climbing past the middle. It's almost
    impossible to get them to climb. This happens at about 5000 to 7000 rpm.
    4) Not sure what an acceptable level of slide up/down jitter is, but my
    slides occolate up and down about a centimeter when I try to apply 50% or
    more throttle until it climbs up. This jitter is probably about 20/30 times
    a second.
    5) All 4 slides act the same when the throttle is blipped, they all
    stutter the same way.
    6) I think the carbs might be a set of BS34SS Mikuni's.
    7) The slide/diaphram on the carbs appear to be still within the service
    limit. The rubber isn't super soft and virgin, but there isn't any sort of
    holes in them either. However the slide cap (located on top of the side)
    that sandwiches the the rubber diaphram to the slide had about 1 to 2
    thousands play in them.
    8) When I dissassembled the carbs the first time I found the main needle
    and it's associated parts inside the slide installed in the following order.
    Small nylon dounut washer, needle with e-clip installed, spring seat washer,
    slide return spring.
    9) The bottom surface of the slide has 3 holes drilled in it. The middle
    hole is for the needle, the other 2 are vaccum signal ports for the chamber
    above the slide. All 3 of these holes are exactly the same diameter.
    10) The engine rpm can almost never climb past 7000, At this speed the
    engine stutters and seems to go super lean.
    11) The slides don't seem to be stuck or hanging from mechanical friction.
    And the bearing surface where the slide moves seems good.

    These are my questions.

    1) As a practical rule, how much high speed slide jitter is acceptable?
    None? 1% of the total slide travel? 10% of the total travel?

    2) When installing the needle it appears that parts go in the slide in the
    following order, Needle first, nylon donut, spring seat return, then
    spring. Not the way I found them, which was nylon donut, needle, spring
    seat, return spring. Which is correct?

    3) What size should the 2 signal ports on the bottom of the slide be (Mine
    are about 2-2.5 millimeter each)? Mine are the exact same size as the
    needle diameter. This seem much, much, too large for me. Is it possible
    the prior owner was clueless and overboard these out to make them "more
    responsive"? Anyone know the factory spec to the diameter of these holes?

    4) Do the slide/diaphram assemblies on Katana carbs have a high failure
    rate? Maybe where the plastic slide cap is rivited? Or are they about as
    good as any other CV?

    5) On other CV carbs how big are the vaccum signal ports on the bottom of
    the slides? 10% of the needle diameter? Do they Jitter?

    6) With mid range fuel problems what are most likely things to look for?

    7) Does anyone know, or could you look up for me what the stock needle &
    jet size where from the factory? I don't own a service manual for the bike
    and have no idea what the right needle and jet sizes should be. The needles
    that came with the carb are non stepped for the E-clip. I assume this is
    the was factory needles were, so that people couldn't easly adjust them.

    8) Anyone have any other tips or suggestions?

    Thanks in advance.

    Wesman, Jun 6, 2005
  2. Got a 4-into-1 exhaust system or the stock pipe? Google up
    "kaybearjr@aol +flat spot" to read what I've said many times about the
    bad interaction between an aftermarket pipe and the carbs...

    Got an air box or K&N separate filters? Google up "kaybearjr@aol +air
    box" to read what I've said many times about the advantages of keeping
    the stock air box...

    Got EPA plugs still installed, or have they been drilled out so a
    shade tree mechanic could screw up the idle mixture by turning the
    screw out too many times? Google up "kaybearjr@aol +EPA"...

    Want to know what the stock jet sizes are? Go to an online parts fiche
    like That site will show you the right order of
    parts assembly on the needles...

    Somebody might have installed one of those bogus Dynojet tuning kits.
    You can tell immediately if you have Dynojet's own crude proprietary
    jets, they will have crudely stamped numbers on them. Original
    equipment Mikuni jets will have nicely engraved computer style numbers
    on their precision jets...

    A FactoryPro kit uses Mikuni jets though... has good instructions for how to set up carbs for
    high performance work. But the best advice for many street riders is to
    leave the airbox on, use a K&N filter that fits inside the box, use the
    stock exhaust system that works with the box, and if you just have to
    dick around with the carbs, only tweak the idle mixture screws about
    1.5 turns either way...

    You could be right about somebody having drilled out the "vacuum hole"
    in the slide for better response. Who the heck would know what the
    hole size was? Maybe you could get to be buddies with the part$ counter
    guy at your local $crewzuki $tealer$hip and he might let you poke some
    precision pin gauges through a new slide. Or, you might fork over a
    bunch of $$$ for a new $pecial-ordered $lide, measure it and try to
    return it. Good luck on that one...

    Or, you might call up Dynojet or Factory Pro and tell them you think
    maybe somebody has already drilled your carb slides out and hope they
    will tell you what drill size they include in their kit$ that they
    $ell, as if they were going to actually give away proprietary
    information that they'd gotten by hours of expen$ive dyno te$ting...

    Maybe you could get some plastic tubing of the same diameter and
    superglue it into the hole that you suspect has been drilled out and
    see if the throttle repose improves?
    krusty kritter, Jun 6, 2005
  3. Wesman

    Wesman Guest

    Thanks, between the time I had posted and you had replied, I did find the
    google searches you talked about.

    I think the comments you made are indeed some but not all of my problems.

    The flat spot or bog talked about in the other posts between 5k-7k sounds
    far more tame than what I'm experiencing.
    I almost can't get the engine up over the 7000 rpm mark complete with miss
    fires, but when I do it rocks.

    Now the prior owner did put a fancy 4-1 pipe on, and now that you mention
    it, I did see reverse fuel vapor clouds form behind the carb durning the
    engine 5000-7000 rpm window.

    I also noticed the bike runs much better with the airbox on, but still bad
    enough as to be unusable.

    I think the needles and jets are still factory. The do have the box in a
    box logo on them.

    As to assembly order, the factory drawings show one way but the bike seems
    to run better with the needle inserted first.

    Do you have any knowledge as to what other CV carbs have for the vaccum port
    size? Do the slides on other CV carbs jitter like mad? At the rate mine
    are oscillating, I wouldn't be surprised if the slides sanded the rails off
    in just a few weeks of running.
    Wesman, Jun 6, 2005
  4. Did you find what I wrote about lean surging with the snorkel removed?
    google for "kaybearjr@aol +snorkel"?
    I understand you to be saying that you put the needle into the slide,
    place the plastic washer on top of the e-clip, then you put the plastic
    spring seat on top of the plastic washer...

    That would lower the needle and lean up the mid range from 1/4 throttle
    to 3/4 throttle normal far too much. If you still have the stock needle
    jet, stock jet needle and stock pilot jet, what you say works for your
    carbs suggest that the fuel air mixture screws are turned out several
    turns and the engine is drowning in rich idle mixture...

    What a lot of shade tree mechanics don't seem to understand is the
    overlapping nature of all the lower speed circuits...

    While the throttle is less than fully open, the engine is drawing fuel
    through the idle jets as well as the needle jet/jet needle combination
    orifice. So, what you do to the idle mixture screw affects the mixture
    at larger throttle opening than seems reasonable...

    You can tell the idle mixture is very rich when the exhaust pipes puff
    out black smoke and the sound of the exhaust is "toot-toot, toot-toot,
    toot-toot" at idle...

    When you open the throttle more and the engine gets more air, vacuum
    downstream of the throttle butterflies drops off, and less fuel flows
    through the idle circuits. The engine cleans out and revs up...
    If I knew that information I would gladly tell you...
    The slides on my Kawasaki KLR jump up and down like crazy as the engine
    idles. There is a long amount of dead time between one intake stroke
    and the next...
    krusty kritter, Jun 6, 2005
  5. Wesman

    Wesman Guest

    Thanks again Kitty

    Last night I measured the 2 vacuum ports on the bottom of the slide, They
    came out to be about .098 inches or about 2.5 millimeter.

    Although, I haven't been able to find the stock specs for the holes, I have
    found mention of "hi-pro" re-jet kits that improve "response" that includes
    a .054 inch drill bit to drill the slide.

    Assuming, I did my math right the cross sectional area of the .054 inch hole
    is 1.49 square millimeters
    and the cross sectional area of the .100 inch hole is 5.1 square

    This would make the new hole a 3.5 times "improvement" over stock, NOT. I
    don't think the air fuel could even come close to metering out the needle
    jet flow with holes this far out of wack.

    I can see how some jitter might should/could occur because air is only
    "intaking" 1/4 of the time on a 4 stroke engine. But my current carbs
    jitter with what I likely believe is probably way to much travel.

    I don't know what it takes in the real world, but it would seem to me that
    you don't need the slides to be able to respond much faster than the engine
    can climb in 1st gear.
    Wesman, Jun 7, 2005
  6. I threw all the extra parts and drills into a box after I installed the
    one and only Dynojet kit I ever bought. The drill for the slide lift
    hole was 0.155 inches...
    I suspect you've confused fuel metering with the servo effect of
    "vacuum" lifting the slide...

    The slide lift hole you measured goes up through the slide, and engine
    vacuum supposedly tries to suck out all the air above the diaphragm.
    The diaphragm isn't actually sucked up by vacuum, though, it's lifted
    by air pressure coming into the carb through an oval-shaped opening
    high up on the intake air bell. Air pressure tries to push into the
    round dome on top of the carb, but the diaprhagm is in the way and the
    slide gets lifted by the air pressure...

    If the slide lift hole is drilled out to a larger size and lighter
    springs from a jet kit replace the stock slide springs, the slide just
    lifts quicker, and that pulls the jet needle out of the needle jet
    quicker, when there's less vacuum available...

    The diameter of the hole in the needle jet (that round brass tube the
    needle slides up and down in)affects the mid-range mixture from about
    1/8th to 1/4th throttle opening, then the taper of the needle has more
    Have you checked your valve adjustment recently? What about

    Maybe the intake valves aren't quite seating and air in blowing back
    out of the cylinders. One reason for intake valves not seating is
    carburized oil buildup on the intake valve heads. Worn intake valve
    guide oil seals leak oil and it drips down onto the hot valve head
    where it solidifies. If the valve guide oil seals leak, you'll see
    smoke when the engine first starts, but it won't last very long.

    Carburized oil can be cleaned off the valves by a shock treatment of
    Techron Concentrate. You can get a 12 ounce bottle of Technon at
    Walmart for about $5.00 and 4 ounces in a tank of gas repeated 3 times
    will help clean the vlave heads...
    (Back in the 1960's when we had SU piston type carbs, each piston
    chamber had an oil reservoir in the top, We filled the reservoir with
    motor oil to damp out the carb piston's tendancy to move too much with
    small increases in throttle opening. it was particular nerve wracking
    to try to maintain a small throttle opening on a rough road. Racers
    didn't care about that, though, they wanted the pistons to fly up as
    soon as possible to richen up the mixture. The needle jet and jet
    needle was the only fuel circuit in the whole carb...)

    You might try what I said about obtaining small diameter plastic tubing
    and supergluing it into the slide lift hole. Another thing you could
    experiment with is adding weight to the slides to make them weigh more
    so they won't move as quickly. One guy I know had a Yamaha XS-750D
    triple that he was tinkering with, and he thought that the slides were
    flying up too fast and making the 50% to 75% throttle range too rich.
    He was used to tuning two-strokes and listening to them gargle on
    excess gas, firing every other stroke until enough air flow through the
    carbs would make the engine two-stroke instead of four-stroking. His
    Yamaha triple was a four stroke, but it was so rich it was
    eight-stroking, i.e., firing every other compression stroke...

    I suggested that he add a few large washers to preload the slide
    springs and add weight to the slides, which were round in his carbs...

    But he figured out that his mainjets were ridiculously large. He was
    used to working with Mikuni hex jets, where a #150 jet will flow 1.5
    tomes as much gas as a #100 Mikuni hex jet...

    It doesn't work like that with Mikuni round jets though. They flow fuel
    as the the area of the jet, which is based on a #100 round jet having a
    hole that is exactly 1 millimeter in diameter. A #120 round jet (1.2 mm
    hole diameter) has about 44% more area, so, if there is enough vacuum
    to pull full through that hole, wide open throttle would draw 44% more
    krusty kritter, Jun 7, 2005
  7. Wesman

    Wesman Guest

    That a single hole or Dual port on the slide? for a Zook 600cc bike or there
    I would assume most 4 cylinder bikes would be similar.

    Don't think so, The slide height (cause needle is attached to slide),
    needle, and jet regulates fuel rate for 2nd fuel. But the butterflies and
    slide regulate air. I see how the slide chases the butterfly valves based
    on engine rpm.
    Now I follow, Air pressure greater under the diaphram, and air pressure
    lower above the diaphram.
    Now I see why the air box/stack is even more important. I had wondered why
    the air chamber above slide was ported into the intake port runner, and not
    just vented to the air along the side if the carb. I did occur to me that
    the chamber vent blocked off as much as 10% of the port cross section.

    But wouldn't a smaller slide vacuum port effectively dampen the travel in
    the slide oscilation?

    Nope, cause bike runs like a banshie if I when/if I can get the slides up
    over 7000 rpm. I might look into that.

    Neat trick that I hadn't heard of it before. I can see how that stuff might
    clean the valves/rings/and engine valve seats.

    I got RTV curing in one of the 2 ports on the slide to try and test it as a
    temp solution.

    I've heard that the diaphrams on other slides are glued on, anyone know what
    brand is used for the adhesive?
    Nice info.
    Wesman, Jun 7, 2005
  8. Yes, they do have two vacuum ports. The carbs I Dynojet-kitted were the
    original BST-31SS flat slide CV's that were standard on the original
    GSXR-750. The drill size probably wasn't 0.155 in., as I look at the
    carbs. It was probably 0.112 in. That's the size of the other drill in
    the box.

    If your engine runs so much better with the plastic spacer washer on
    top of the needle clip instead of under it, it just might be that the
    holes in your slides have been drilled too big and the slides lift too
    much with a little vacuum...
    Rather large venturi CV carbs can be used on 4-stroke engines because
    the vacuum slide keeps them from bogging. 38mm round slide CV carbs
    were common on 1100cc machines. That's 275cc pistons drawing through
    vacuum-controlled venturis. The biggest CV carbs I've ever heard of
    were about 50mm, and they were OEM on the Honda VTR-1000F, with 500cc
    pistons. If you look at, you'll probably see that the
    VTR carbs are flat slides.

    There's more turbulence downstream of a flat slide, so less vacuum will
    suck more fuel...
    I never saw a carb where the diaphragms were glued in place. It seems
    like it would be too easy to tear a diaphragm while working on a

    One thing to look at if you have the tops off the carbs again is that
    some carbs have a little port and a small o-ring that is held down by
    the carburetor top. I think it's a vacuum port. Shade tree mechanics
    lose that little o-ring when they go into their CV's for the first
    time. Check to see if there's a little o-ring or
    krusty kritter, Jun 8, 2005
  9. Wesman

    Wesman Guest

    Looks like I'll have to back and restudy the carb.

    Thanks for the assistance. You where great help. When I find out more,
    I'll let you know.
    Wesman, Jun 8, 2005
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