Hard starting 1985 GPz900R (C model)

Discussion in 'Motorbike Technical Discussion' started by messenger1, Nov 3, 2005.

  1. messenger1

    messenger1 Guest

    Greetings folks. I am having some difficulties starting my old GPz lately.
    After it sits long enough to cool off (3-4 hours @ 15 Celsius) it is almost
    impossible to start. Thus far I have, changed the plugs (old ones looked
    fine) made and installed new plug wires (old ones were a bit corroded where
    they screw in to the terminals), cleaned the carbs (they were spotless)
    checked the compression (148-152-150-149) installed a new battery, measured
    the resistance of the coils (within specs) to no avail, checked the petcock
    ( works fine), and checked for air leaks at the carb boots (none). It will
    kick once immediately upon the first hit of the starter button, with the
    choke applied, as though it was going to start, then just crank forever..
    The first time it happened I just cranked the heck out of it (sorry starter
    but had a buddy impatiently waiting to go for a ride) till I had to boost it
    to continue, still nothing, then I did the unthinkable and used some <I know
    its bad> quickstart and eventually it caught and started and ran fine.Once
    its running it runs like a new bike, although it surges a bit at idle, that
    is, the idle sometimes, not always, fluctuates between 1500 and 2000rpm,
    other times it idles smoothly at about 1500. The next time it happened it
    was a bit cooler out in my garage, about 6 degrees Celsius, so after an
    unsuccessful 2 or 3 minutes or so of cranking, I put the cover on the bike,
    placed a small heater underneath and let it warm for an hour...damn thing
    sparked right up.This bike can't be that cold blooded (can it?). My friend
    has a ZL1000, essentially the same engine, his bike sits outside and it
    sparks right up every time. Help! I'm pulling my hair out here, any

    Tom B
    1983 GPz550
    1984 KZ1105
    1985 GPz900
    messenger1, Nov 3, 2005
    1. Advertisements

  2. messenger1

    skimmer Guest

    Got diaphragm (CV) carburetors?

    Your carburetors probably do not have a real choke with a flat plate
    that makes the carburetors suck harder when its closed. Instead, they
    probably each have a small starting enrichener valve. It's just a
    plunger in a hole and when you move the "choke" lever or knob to the
    full ON position, you are just letting air bypass around the throttle

    The enrichener has its own starter jet in the float bowl and it's
    possible for the fuel air mixture to be as rich as 1:1, but there's
    still not that much air flowing through the tiny hole that's around
    1/4th of an inch in diameter.

    If your idle jets have gotten plugged up over a period of time, you or
    a previous owner probably noticed the idle RPM was too low and you or
    the other guy probably thought it was a simple matter to just turn the
    idle RPM up. That was a wrong move. If the idle jets are plugged up and
    the throttle butterflies are adjusted too far open, the starting
    enrichener cannot suck enough fuel out of the float bowl because there
    just isn't enough vacuum available.

    Even if the engine should fire up for a second, the moment you twist
    the throttle grip,
    vacuum totally falls off and nothing gets sucked through the idle jets,
    which are plugged anyway.

    You want those butterflies closed as far as possible. When owners of
    old British singles tried to kickstart their engines, the first thing
    they would do is turn the idle RPM screw all the way out and close that
    throttle all the way to make the carburetor suck gasoline out of the
    float bowl.

    So you need to clean those carburetors from the inside and then
    readjust the idle RPM to the owner's manual specifications. If you
    don't have the owner's manual there is probably a decal under the seat,
    on the back side of a side cover, on the air box, or even on the chain

    Your engine runs on the idle jets a lot of the time when you're just
    cruising down the highway. You'd be amazed at how little throttle you
    actually need to use. The idle jets and idle passages get gummed up
    when the motorbike sits around unridden for a few months. Gummed up
    jets and passages stop the flow of fuel to the engine when you open the
    throttle and you'll hear the exhaust go "piffle-piffle-SNAP!" or even
    as you roll the throttle off.

    When the idle jets are plugged up, the engine is hard to start, takes a
    long time to warm up, and acts very "cold blooded". It wheezes and dies
    when you open the throttle.

    Further confirmation of this diagnosis is that you say the engine RPM
    "surges" while you're riding. That also points at plugged up idle jets.

    The easy solution is to go down to Wal*Mart or any good auto parts
    store and buy a 15 ounce can of Berryman's B-12 Choke and Carburetor
    Cleaner. Add 3 or 4 ounces to a full tank of gasoline and go for a slow
    ride. Watch for cars coming up behind you!

    You want to ride slowly so the engine has to suck the mixture through
    the idle jets. After several miles of running, the idle RPM will
    increase and you will have to turn the master idle knob down. It may be
    underneath the carbs or on top. It will probably be between the two
    carbs on the right side of the engine as you sit on the motorbike.

    Once you have cleaned out your carbs, the engine should start with the
    "choke" all the way ON and you shouldn't have to touch the throttle
    grip at all. You should be able to put your helmet and gloves on while
    the engine warms up, without having to fiddle with the choke to keep
    the RPM from getting too high or too low.

    If the idle jets and passages are really plugged up and you have to
    remove the EPA anti-tamper plugs to spritz out the hole around the idle
    mixture screw, I've described the drilling out process and readjustment
    of the screws about a bazillion times here in this forum, you can
    google up EPA and find it.
    skimmer, Nov 3, 2005
    1. Advertisements

  3. messenger1

    messenger1 Guest

    Thanks for the reply. yup they are Keihin CV's. I checked and recleaned the
    idle jets. Damn thing starts now but still dies with no throttle applied
    till it gets warm unless I screw the idle adjuster way up, then when warm it
    races and surges. Now however, when I flip the choke lever all the way to
    the on position, not only will it not start, but when I do so when its
    running, it will die. ??????
    Of note, I got so stressed trying to get those godforsaken boots back onto
    the stock airbox that I gave up and slapped on a set of pod filters I had
    laying around. It was running a bit lean so I installed a set of larger
    mains (I drilled an old set out very marginally larger than the stock ones
    since these carbs have jet needles with no clips to raise or
    lower...bad/good/really dumb idea??) and now it runs great when warmed
    up,( lost that stutter at 4000rpm I picked up with the 4-1 Muzzy) but does
    not seem to have the same initial early throttle response although it still
    has lots of power down low. Plugs colour is a nice even tan now after a
    chop, but the "choke" inoperativeness question remains.
    regards and TIA
    Tom B
    messenger1, Nov 6, 2005
  4. messenger1

    skimmer Guest

    Surging indicates lean mixture.

    Did you drill out the EPA anti-tamper plugs so you can spritz out the
    idle passages with aerosol carb cleaner? I've describe the process
    about 100 times, but basically when you squirt carb cleaner through the
    idle mixture screw holes, it should come out the idle mixture port, the
    transition ports next to the throttle butterflies, the idle jet and the
    pilot air jet. If the carb cleaner doesn't squirt out one of those
    holes, cover up the other holes with your fingers and keep quirting
    until it does.
    All the way ON is up? What you're doing is opening a bypass air valve
    in a passage that goes around the closed butterfly. It's an idle
    enrichener, not a true choke. It's supposed to suck gasoline straight
    out of the float bowls through its own starter jet.

    Maybe the starter jet is plugged up? What do the plugs look like when
    the engine dies? Are they all sooted up or clean?
    Pods will cause the engine to run lean. It's hard for most shade tree
    tuners to figure out the round jet sizing system. I've explained that,
    too, at least 100 times. A #100 round jet has an orifice that's 1.0
    millimeters in diameter. If you install a #120 main jet, it doesn't
    pass 20% more fuel, it has like 50% more area in the orifice because
    the diameter is 1.2 millimeters. You can work out thearea of the
    orifice using the pi times radius squared formula to see that a 120
    main jet should be plenty big.
    Drilling out the main jets is not a good idea unless you have access to
    some very precision drills. The factory can drill those things out very
    precisely. The difference between a #100 main jet and the next size (a
    #102 main jet) is about 0.0008 inches.
    It sounds to me like your idle passages are still all plugged up. Just
    as you open the throttle, the butterflies should be uncovering three
    transition ports that give the engine the shot of fuel it needs to
    accelerate, since the vacuum slides will not have been lifted high
    enough to get the tapered part of the needles out of the hole.

    Plugs colour is a nice even tan now after a
    Plugged up starter jet?
    skimmer, Nov 7, 2005
  5. messenger1

    messenger1 Guest

    There are no plugs covering the idle jets on mine, maybe they have been
    changed by a previous owner? I pulled them, took great care to remove the
    springs-washers-orings and then squirt cleaner and blow compressed air
    through them .

    I should have been more specific....The "choke" lever is bar mounted, when
    rolled to the fully on position it actuates the enrichener.
    Plugs look great now.
    I received a fairly complete set of numbered precision bits (finally a
    really usefull b-day gift!), I sized the original jets (0.042) and then used
    a bit one size larger (0.0425) so oops guess that was a bad idea, good thing
    I kept the originals unmolested!
    Now heres my newest dilemma....where in the heck is the "starter" jet?????
    I've identified, cleaned, and gone over the main jet, the needle jet, the
    jet needle, the pilot fuel jet but cannot find a starter jet!

    thanks in advance!
    Tom B
    messenger1, Nov 7, 2005
  6. messenger1

    Leon Guest

    When I had a GPZ900R a few years ago (an A1), I didn't bother with the
    old rubbers on the carbs, and replaced them when I had to remove the
    carbs to fit a new throttle cable. New rubbers are still tricky to get
    on properly, but *much* easier than old hardened ones. It helps if you
    have small hands, I was able to get mine inside the airbox.

    You could try the Yahoo GPZ900R group.

    Leon, Nov 7, 2005
  7. messenger1

    skimmer Guest

    I looked at the carburetor parts fiche for the 1985 900 Ninja. Is that
    what you have? Is your machine water-cooled, with 4 valves per cylinder
    and has the cam chain on the right hand end of the engine?

    There was a selection of main jets listed, from #128 to #140.

    A main jet that is only 0.042 would be a #106, which Keihin doesn't
    make, but they do make a #105. That's a very small jet that would work
    OK on a smaller, mildly-tuned engine, like maybe a 500cc or 750cc

    I can't imagine your engine running well at full throttle with anything
    smaller than a #125 main jet, unless you ride at 5000 feet above sea
    level all the time
    If you look at a parts diagram and don't see a removable starter jet
    listed, that's because it's pressed into the carb body or even into the
    float bowl.
    skimmer, Nov 7, 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.