95 GSXR 750 jetting advice needed

Discussion in 'Motorbike Technical Discussion' started by Pete, May 29, 2005.

  1. Pete

    Pete Guest

    Hi, I'm helping my brother rejet his Suzuki. It has a K&N filter (in the
    stock air box) and a yoshimura exhaust. (not sure if it's a slip on or not)
    The engine runs good..no clanks or ticking noises or blue smoke. The bike
    idles strong, but it has a flat spot after half throttle, but seems to run
    well up to half throttle. I don't know if the previous owner has tryed to
    rejet it. He just bought the bike a few days ago and we are pricing jet
    kits. From what i've heard it's tough to get all the flat spots out when
    rejetting a bike. The dealer here told us the bike uses a #115 stock jet so
    we can tell if it's been worked on already. The dynajet kit for a 95 GSXR
    750 only comes in stage 1, thats suppose to be with a stock airbox and stock
    exhaust. Does the slide needle on this bike have the notches to raise it's
    height? Are there any tips to rejetting, like checking the plug color from
    different loads and speeds? I would like to get the bike to run as good if
    not better than stock. I have a stock 90 CBR 1000 and i've raced other CBR
    1000's with headers and jet kits that were installed at the dealers and
    their performance wasn't all that great. I would imagine their problem was
    in the jetting. Any advice would be appreciated.

    Pete, May 29, 2005
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  2. Before you start wasting $$$ buying bogu$ jet kit$, find the EPA
    anti-tamper plugs and drill them out. Look underneath the carburetors,
    forward of the butterflies with a mirror. Google for "[email protected]
    +EPA". If the plugs are missing, they've already been drilled out. Is
    somebody has drilled the plugs out already, they've probably turned the
    idle mixture screws out four or five turns and that's too many. The
    optimum number of turns is somewhere between 1.0 and 3.5 turns. If you
    have to turn the idle screws in to less than 1.0 turns, the idle jet is
    too large. If you have to turn the screws out to more than 3.5 turns
    the idle jet is too small. Nothing happens after 3.5 turns...

    While you have the carbs off the engine to drill out the EPA plugs,
    squirt carb cleaner through all the passages and do your basic idle
    mixture adjustment vs. specified idle RPM adjustment and synchronize
    your carbs before you buy some expen$ive bogu$ jet kit...
    Aftermarket jet kits are bogu$ bull$hit. Riders hear or read about some
    "trick" new exhaust system in the motorbike magazines. They get free
    performance accessories and write about their screwups as though
    whatever system they installed was the most wonderful thing in the
    world, regardless of whether it was a big clusterfuck or not. If they
    told the truth, they wouldn't get any more free shit to write their
    fairy tales about...

    The magazines are whoring after the ego market now. There is a lot of
    income out there, and the magazine$ and the aftermarket performance
    part$ manufacturer$ are grabbing a lot of riders by their egos, telling
    them that they just can't belong to the wonderful world of motorbike
    riding if they don't have the most expen$ive and powerful machinery
    they can get...

    Newbie motorbike riders figure that they can enter this wonderful world
    of performance by buying some aftermarket jet kit that's supposed to be
    a "bolt on solution" to some performance problem they never knew they

    Riders who have any kind of luck with their bogu$ bull$hit jet kit will
    be getting more
    "performance" for the simple reason that they now ride the shit out of
    their machine.
    If they hear somebody badmouthing Dynojet, they will go nose to nose
    with the guy telling the straight story, because it would hurt their
    ego to admit they'd been fucked
    out of their money by Dynojet...
    You CANNOT re-jet a bike with a 4-into-1 exhaust system to get rid of
    the flat spot between 5000 RPM and 7000 RPM. We've had the flat spot on
    our sportbikes for over 30 years now, and the solution isn't
    re-jetting, it's Yamaha's ExUP system, or Suzuki's SET system, or
    whatever Honda and Kawasaki are calling the computer controlled exhaust
    throttling valve in the collector of the 4-into-1...

    Or, you can install zero overlap cams that eliminate the overlap period
    when both intake and exhaust valves are open, and kiss off the high RPM
    benefits of a
    4-into-1 header system...

    The throttling valve breeaks up the undesireable positive pressure wave
    that comes from the collector and flows right through the cylinder
    duting the overlap period and pushes incoming air right back out of the
    About the biggest main jet you could use on a GSXR-750 is a #130.
    Between the #115's and the #130's there are 6 jet sizes and if you buy
    enough jets to experiment with, you need 24 jets at $4.00 each. So
    there's $96 worth of jets and 20 of those jets aren't going to be the
    jets you're wanting and you can't take them back :-(

    You might hang around your friendly local racers and buy them coffee
    and butter them up by asking questions and listening to whatever they
    have to say, and maybe they would loan you some of their excess brass.
    Every old time racer has brass. They bought it when they didn't realize
    they'd only need four jets to make their bikes run right. They just
    didn't know *which* four jets they needed...

    Maybe it would be cheaper to just buy the Dynojet kit and accept that
    they are making their own jets that don't correspond to Mikuni jet
    sizes and their own needles that don't corespond to Mikuni needle
    sizes, and maybe they give you two drills, one to drill out the EPA
    plugs and one to drill the "vacuum" hole in the slide bigger for faster
    throttle response...

    You see, Dynojet kits are proprietary kits, developed to their own
    secret specifications. A smart guy could reverse engineer his own kits
    from measuring kits that he bought from Dynojet or from Factory Pro,
    but he would need a machine shop to make his own jets and needles...

    Or, a smart guy could just buy the Sudco Mikuni carburetor parts and
    tuning manual.
    There's a whole shitpot of free info in the Suco International Catalog
    and on the factorypro.com website too. Where did they get all that
    information? From Mikuni and Keihin, or course. They want to $ell lot$
    of carburetor$, so they give away FREE information...

    But Dynojet want$ to keep that free information $ecret, so they can
    $ell you a jet kit...

    Dynojet kits are developed on their own dyno and the horsepower outputs
    are corrected to sea level standard temperature, pressure and humidity
    I don't know what altitude you live at, but if you live at 5000 feet,
    you will lose 14% of your sea level power just due to the altitude and
    more than that if the temperature isn't 59 degrees F. So, if you live
    in the mountains, don't have great expectations from rejetting a
    smaller engine, just buy a larger bike...

    After I forked over my precious hard-earned cash for the one Dynojet
    kit I ever bought, I called up Dynojet's technical help line and asked
    them why I had the terrible flat spot in the 5K to 7K range. They told
    me that I should just accelerate through that RPM range at full

    I told them I couldn't do that, because I was riding in tight canyons
    where I was trying to get out of 2nd gear and ride in 3rd. But ever
    time I'd shift up, the engine would drop back into the flat spot. The
    Dynojet tech finally admitted that it was impossible to rejet my carbs
    to get rid of the flat spot...

    He said that other riders were buying later model carbs with larger
    venturis to overcome the bad flat spot. It seems that a carburetor
    doesn't know which way the air is flowing through it. During the flat
    spot, air flow reverses through the carb and it picks up a shot of gas.
    When the air flow turns around and goes the right way, it picks up
    another shot of gas. So the mixture is too rich on each side of the
    flat spot and too lean in the middle of the flat spot...

    Buying $500 worth of larger venturi carbs would reduce the flat spot
    rich/lean/rich condition by reducing vacuum in the carbs. Dynojet's
    bolt on tuning kit solution didn't work, but they still had my money. I
    didn't solve the problem until I found a guy with a set of used 33mm
    smoothbore carbs and a nice exhaust system for $325...

    There's some old saying about "Screw me once, and I figure out you're a
    crook, screw me twice and I must be stupid, because I couldn't figure
    it out the first time."

    Dynojet got their one shot at my money, and they blew it...
    I looked at the partsfish.com web site and found a 1992 GSXR-750. But
    no 1995 carburetor drawing. The 1992 had needle clips...
    You have to start with your fist full of $96 worth of main jets and
    guess what
    ones will work. Maybe if you haven't unwrapped some of them you can
    take them back to the $tealership when you figure they aren't needed?

    Anyway, you start with new spark plugs of the correct heat range and
    use whatever jet you guesstimate will work, and you do high speed full
    throttle runs on a nice dry sunny day when the temperature is about 59
    degrees and the barometric pressure is 29.92 inches of mercury...

    Swap jet until you find the jet that lets the engine pull the hardest
    to redline and you run the engine at redline for a few miles. You won't
    hurt your Gixxer motor by running at that high RPM, it can take it, it
    has a very short stroke, it hardly knows it's turning that fast...

    Then you pull the clutch in, and hit the kill button and coast to a
    stop and you pull the plugs out and look down the spark plug for a thin
    black ring of sooty carbon way deep down inside. Maybe that ring is
    1/8th of an inch wide. If the soot comes way up the spark plug, almost
    to the center electrode, the main jet is too big, or the weather is too
    hot and humid...

    If the engine pulls harder at high rpm when cold and less hard when
    fully warmed up, the main jet is too large. Install a smaller main jet
    and retest until you find the main jet that pulls the hardest at high
    rpm when fully warmed up. This is supposed to be done first, before
    trying to tune the jet needle and the idle mixture...
    If the bike doesn't pull well at high rpm when cold and gets only
    slightly better when fully warmed up, the main jet is too small.

    Don't worry about the needle position and the idle jet setting while
    you're selecting the proper main jet. When you are running full
    throttle, the idle jets and needle have no effect on the mixture...
    That's not surprising. Most $tealer$hip mechanic$ are "procedure
    trained" putzes. They went to a tech $chool, they went to the
    di$tributor$ $chool and and got a bunch of certificate$ and they do
    procedure$ on customer$ motorbike$
    all day, but they don't seem to know the first thing about what's
    happening with
    jetting, unless they are $elling you a "performance kit"...
    krusty kritter, May 30, 2005
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  3. Pete

    Pete Guest

    Thanks a lot!! for all your time and info! I had thought about just buying
    a few dif sizes of jets after I looked in the carbs. I'll look for the EPA
    plugs first and then start on the main jetting. The idle screws look like
    they have been backed out more than normal. (though i haven't counted the
    turns on them yet. I'll let you know the results..

    Thanks again,
    Pete, May 30, 2005
  4. Pete

    Pete Guest

    We drilled the mains to 1.18mm and turned all the EPA screws to 2 turns.
    It stopped the power drop off after half throttle and seems to pull a little
    harder throughout the power band. But i'm getting a little pop and crackle
    during deceleration which isn't to bad. Seems to be idling just a little
    rich..no black smoke, but you can smell some gas fumes. When we tear it down
    to try the #125 mains (when they come in) we will try 1 or 1 1/2 turns on
    the EPA screws. Oh the main jets' needles didn't have adjustments for
    height. Just a plastic washer under a circle clip.

    Pete, Jun 13, 2005
  5. Any progress is good, it shows you're headed in the right direction...
    If the idle jets are a little small, that causes a fartPop! sound and
    the engine almost stalls if the idle mixture is too lean. You can get a
    crackling rumbling sound from rich mixture...

    Dual lead coil ignitions fire a waste spark into the cylinder that's on
    exhaust stroke. If your overall mixture is too rich, when you roll off
    the throttle and coast, you can hear the rich mixture rumbling in the
    exhaust pipe. One spectator told me that my GS-1100 with a Kerker pipe
    and overly rich carburetion sounded "awesome" coming down the hill at
    Willow Springs with small throttle opening...

    Or you might have spark plugs that are one heat range too hot, or maybe
    the original owner installed an ignition advancer to help the

    Ignition advancers increase the combustion pressure at lower RPM. Since
    the ignition advance curve is built into the ignition controller, a 5
    degree advancer would make the static advance about 15 degrees instead
    of 10 degrees and full advanced would be about 37 degrees instead of 32
    The ignition control box doesn't know this, though, the aftermarket
    ignition unit is one thing, the intended advance curve is another...
    If you quickly close the throttle, does the engine take several seconds
    to return to a normal idle? The idle mixture is too rich in that

    If the idle mixture is really rich at idle, the exhaust sound will be
    "toot-toot, toot-toot, toot-toot", and the engine will blow out the
    excess mixture when you open the throttle...
    Sounds good to me...
    You can get 0.5mm thick washers with 3mm ID holes and stack a few under
    the needle clip. But that will have the greatest effect between 1/2 and
    3/4 throttle...
    krusty kritter, Jun 13, 2005
  6. Pete

    Pete Guest

    Well I rejetted the mains to #125 and the bike runs a lot better and I can
    feel any flat spots. And the new plugs we put in show a lite tan color with
    no smoke from the exhaust...But the bad part is that the GSXR 750 will
    almost beat my CBR 1000... maybe I should put the old jets back in!!
    Thanks for all the advice from the newsgroup!!

    Pete, Jul 4, 2005
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