hard starting GPz900

Discussion in 'Motorbike Technical Discussion' started by NA, Dec 18, 2005.

  1. NA

    NA Guest

    Maybe a weak/marginal battery? Time for a replacement?
    NA, Dec 18, 2005
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  2. NA

    CK Guest

    Refusal to start when cold is usually an indication of a lean starting
    mixture or that your spark plugs are really old and eroded and it takes
    more voltage to fire a cold spark plug when the center electrode has
    rounded off after many, many, many miles.

    I used to take the spark plugs out of my old 1960's Hondas and Yamahas
    and heat them up over the gas burners of the kitchen stove and then the
    engine would finally start and I was mystifed as to why heating the
    plugs helped. I knew nothing of the voltage requirements necessary to
    fire a lean mixture and the random nature of fuel/air
    mixture being near the spark, should it ever actually occur. :-(

    Of course, you don't want to have to remove your spark plugs and heat
    them up every morning, and you don't want to have to keep your engine
    warm from an outside source, it just invites a fire to start.

    If you pull the cap off your spark plug and you see a bright blue spark
    at least 1/4 of an inch long jumping to the plug, you should have
    enough voltage to start the engine.

    It hasn't gotten down to freezing around here yet, but I have already
    begun hooking up a trickle charger to my truck's new battery to get
    maximum cranking time when I start it up a few times a week.

    But I suspect your problem is actually lean mixture. If your idle
    mixture screws were adjusted properly, you wouldn't need to touch the
    throttle handle at all, you would just put the "choke" lever to the
    maximum position and push the starter button.

    I'm sure that you know by now that your carburetors don't have real
    "choke" plates, they have individual starting enricheners, which are
    just little valves in a passage that bypasses the main venturi. When
    you move the "choke" lever to the ON position, a plate slides to one
    side, opening all four starting plungers. The plungers allow air to go
    around the closed butterflies and the air is able to suck fuel right
    out of the float bowls.

    There is no room in the tiny passages for a tiny little venturi, the
    vacuum from the engine is enough to suck gasoline up into the starting
    bypass passage just like sucking milk through a straw.

    If the throttle butterfly plates are slightly open, there is less
    vacuum at the starting enrichener, and no gasoline gets sucked out of
    the float bowls and the engine won't start.

    So the first thing to do is to turn the master idle knob all the way
    counterclockwise so the butterflies are closed all the way to maximize
    vacuum at the starting enrichener outlet that's downstream of the

    And, I also recommend studying that "choke" linkage that pulls the four
    plungers open.
    Some Kawasakis have a "fast idle cam" or "chock" that holds the
    butterflies open slightly, but only while the "choke" is on. So check
    to see if one of those metal parts between the carbs holds the
    butterflies open, and you'll definitely know not to turn the throttle
    when "choking" the engine.
    I suppose by now you have drilled out the plug covering the idle
    mixture screw. The mistake newbies make when they try to adjust the
    idle mixture screw is that they turn it too far counterclockwise,
    expecting the idle speed to increase.

    When the idle speed doesn't increase, they may turn the idle mixture
    screws so far that the idle speed actually decreases. The exhaust sound
    becomes dull and thudding,
    and smells rich, and the engine stalls from idling too slow, so they
    turn the master idle knob to increase the engine speed.

    Now that opens the butterflies too far, uncovering the three transition
    ports prematurely. When the engine is warm and the rider blips the
    throttle, the idle speed hangs up at 4K to 5K and it takes a long time
    to return to normal idle speed.

    If the amateur tuner has turned the idle mixture screws excessively far
    out, the engine sound at idle is "toot-toot toot-toot toot-toot" and
    the excessively rich mixture doesn't clear out until the throttle is
    opened a lot. The throttle response is poor and the engine tends to run
    too fast at closed throttle, pushing the rider deeper into turns when
    he really wants to be slowing down using engine compression.

    The correct method of setting the idle mixture screws is to turn the
    master idle knob so the engine is idling at the specified RPM, then
    turn each idle mixture screw CLOCKWISE about 1/4 of a turn at a time
    until the RPM increases, turn the master idle knob down, turn the
    screws in, turn the master idle knob down, and continue in equal stages
    until the engine begins to run rough from lean mixture, then back each
    idle mixture screw out 1/8 or 1/4 of a turn.

    It's nice if you have an accurate tachometer that will resolve RPM down
    to 50 RPM, or you have vacuum gauges to indicate that each cylinder is
    pulling the same amount of vacuum at the specified idle RPM.

    And, you'd be in heaven if you had an exhaust gas analyzer you could
    plug into each of the four exhaust pipes. You could set your emissions
    right on what the EPA specifies. You'd be back to stock and idling very
    lean and the engine would be hard to start on cold mornings and would
    be slow to warm up.

    That's why we started drilling those damned EPA plugs out in the first
    place, 25 years ago.

    But, if you get your idle mixture screws set right and your master idle
    knob set to the specified RPM on the EPA decal, your engine should
    start on full choke, without touching the throttle grip.
    Well, I would bet that makes the Dynojet folks happy. It solves their
    cash flow problem, yannow. Dynojet makes their money off of the
    motorcycle owner's self esteem (he wants his machine to be as fast or
    faster than the other guy's identical machine) and pride (owning a
    sportbike means that you have to be interested in high performance,
    that you can't be satisfied with a stock machine and buying a jet kit
    gives you something to talk about when you are at the motorcycle

    But, does the engine start any better with the K&N filters removed?
    Have you over-oiled the filters? That's a common problem, riders spray
    that K&N oil onto the filters until they are just dripping oil, and
    that chokes the engine by restricting airflow and it really is too rich
    with over-oiled filters.

    So far as the 4-into-1 Muzzy is concerned, it would do NOTHING to make
    your mixture too rich at any point in the RPM range, but a 4-into-1
    pipe does cause leaness from 5K to 7K, it's the well-known flat spot
    where flow reversal takes place as the engine returns an adverse
    pressure wave that blows exhaust gasses backwards through the intact
    tract during the camshaft overlap period.

    Your starter cannot turn the engine 5K to 7K RPM, so the Muzzy won't do
    anything as regards starting the engine.
    So how many turns out are your idle mixture screws, anyway? Three or
    four turns? Try
    1.5 turns out on all of them and work towards 1/4 of a turn. The idle
    jets on CV carbs are usually HUGE, so the idle mixture screws have to
    be turned out only a little bit.
    CK, Dec 18, 2005
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  3. NA

    messenger1 Guest

    This bike is driving me nuts. Its a 1985 GPz900 that up till about 3 months
    ago ran fine, started fine and gave me no problems. Now, it will absolutely
    NOT start unless its warm (being in Canada in fall and winter that entails
    covering it and putting a space heater under the cover) I have cleaned and
    adjusted the carbs 7 times now. In desperation I bought a dynojet kit in
    case the 4-1 Muzzy or the K&N pod filters were affecting the mixture enough
    to prematurely foul my plugs. Plugs have been replaced each time I clean the
    carbs (even though they looked fine). Each time I clean and reassemble the
    carbs, the bastard fires right up, runs like new, and even idles fine.
    (although, it will die if full choke is applied quickly) After it cools, it
    AGAIN will not start cold.I have installed an inline fuel filter, and it
    looks clean each time I remove the tank/carbs. Each time I remove them, the
    carb bowls are full of fuel and they don't piss out while installed.The
    intake boots are fine, no vacuum leaks, the engine has lots of compression
    and when its finally running it starts and shuts down great.

    In desperation I have even tried an old kawy Z trick of wiring a relay up to
    provide a full 12+ volts at the coils, no help.

    Any help folks??? I'm at my wits end and have been eyeing a gas can, a
    length of yarn and a wide open spot in my backyard and calculating the odds
    of my neighbors calling the cops while it burns there. Please don't let this
    nice old bike suffer a tragic end at my deranged hands!

    While I was in at the local Kawy/Duc dealer buying new o-rings and washers,
    I spotted a 2003 Z1 NOS for 7g's. That may be enough to push my unstable
    self right over the edge.....


    Tom B
    messenger1, Dec 18, 2005
  4. NA

    CK Guest

    There is only one size for the idle mixture screw. The parts diagram
    calls the idle mixture screw a "pilot AIR screw" but I believe that's
    the wrong nomenclature, since there is a tiny washer and an o-ring on
    the screw. Pilot air screws wouldn't need the o-ring and washer.
    Problem with the Dynojet main jets is that the sizes don't match Keihin
    and Mikuni jet sizes exactly, so you cannot compare them to original
    equipment jets.

    And the design is different too. The Dynojet main jets are just a
    straight drilled hole. The OEM jets have a venturi shape to them. But
    main jets don't influence starting.
    Yes, 360 degrees is one full turn of the screw. It turns out that the
    900 Ninja used fairly small idle jets, they are # 35, and I was talking
    to another guy who said that Mikuni had used # 35 pilot jets in his
    1200cc Bandit and that he also had to turn the idle mixture screws out
    2.5 turns.

    The only other things I can think of is that the starter jet that's in
    the float bowl or carburetor body is plugged up. The starter jet isn't
    shown on the parts diagram, it's probably permanently cast into the
    carb body or the float bowl.

    And, does turning the petcock to the PRI position help when you start
    the engine from cold? It's possible that you just don't have enough
    vacuum when you're cranking the engine on a cold day, or maybe the
    little rubber diaphragm in the petcock leaks vacuum, or the shut off
    valve that the diaphragm is supposed to open is stuck?
    CK, Dec 19, 2005
  5. NA

    messenger1 Guest

    sorry forgot to mention, also replaced the battery and checked the charging

    Tom B
    messenger1, Dec 19, 2005
  6. NA

    messenger1 Guest

    I've actually pulled these out and off the carbs twice now, so I could
    spritz out the openings with carb cleaner and lots of compressed air. They
    move freely and the passages are definately crud free at this point.
    I have checked that, with the idle adjustment knob turned all the way out
    the butterflies are completely closed.
    This bike arrived in my possession sans the plugs, the carbs were supposedly
    cleaned and synced "professionally" by the previous owner....perhaps the
    wrong sized idle mixture screws?
    It starts the same with them on, oiled or un-oiled (have a spare new set)
    just runs crappy with them removed. As for the dynojet kit, it was a
    desperation thing, I thought the kit would contain a complete range of the
    required needles & jets etc instead of just a new jet needle and a selection
    of mains :(
    Tried 1.5 originally, as always after a cleaning/adjustment the bike started
    fine the first time but popped and spit through the carbs, On subsequent
    cleaning and adjustment sessions I have set them at 2, 2.25, 2.5 and 2.75
    turns out. The bike idles best with them at 2.75 turns out from fully
    seated. Maybe I'm over thinking something? I consider a turn out 1 complete
    360 degree revolution of the screw, is that correct?

    thanks for the input/advice/help!

    Tom B
    messenger1, Dec 19, 2005
  7. NA

    messenger1 Guest

    hmmm, I haven't tried starting it in the prime position since my original
    carb cleaning session, I never considered the petcock as it appeared to be
    working fine.....I will definitely check it.

    Tom B
    messenger1, Dec 19, 2005
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