help! gas leaking from carbs on ninja 250.

Discussion in 'Motorbike Technical Discussion' started by Sean J Kelly, Aug 22, 2005.

  1. Sean J Kelly

    Sean J Kelly Guest

    hey everyone.

    recently i helped a friend of mine get his first bike, a 1996 kawasaki
    ninja 250. it hadn't been ridden for 3 years, but had recently been
    serviced at the local dealership. it received a new rear tire, fluids,
    and a "carb overhaul".

    i test rode the machine before he bought it. it seemed to stumble a bit
    off-idle, but ran fine once it cleared its throat. i mostly stayed
    between 5-10k rpm, and had no troubles.

    once he'd done the title transfer, i rode it back to my garage. again, at
    moderately high engine speeds. no problems. i parked it and it sat for a
    couple of weeks.

    a few days ago, i left to ride it over and deliver the bike to him. i had
    a really rough time getting it started - it didn't want to idle. after
    running it on the choke for awhile, i turned off the choke and adjusted
    the idle speed screw until i got a reliable idle - i had trouble getting
    it to idle below 2000rpm or so.

    i took it to a gas station and filled the tank, thinking that old fuel
    could be causing some of my problems. when i was filling it, i could
    really smell gasoline - i looked and saw a puddle forming underneath the
    bike. i attributed this to a bad petcock o-ring (it will leak if you push
    on the knob the wrong way). i eventually got the bike started, after
    quite a lot of cranking, and rode it up the street to my friends' place.

    we let it idle for a bit, and then i noticed leaking gasoline again.
    under closer inspection, it was not coming out of the petcock. the
    gasoline is coming out of one (or both) of the carbs. it's not coming out
    the float drain screws. it seems to come from a level above the float
    bowl on the inside of the pair of carbs.

    one more important (i think) clue - the bike will sit just fine and hold
    gasoline. until you start cranking the motor. after a few seconds of
    cranking, it spills the gasoline.

    so, i think that there are a couple of problems here.

    one, the plugs are almost certainly fouled from all the attempted starts.
    i'll have to pull and clean or replace them.

    the bigger one is of course the gasoline running out of the carbs. it
    comes out at a pretty good rate - enough to form a stream instead of
    individual drops.

    can you think of any likely possibility before i pull the carbs off and
    apart? i don't really understand how these carburators operate, as i've
    never taken one of these pairs apart. they have a few extra hoses above
    the cylinder-side boot, and one that runs back to the petcock besides the fuel
    line (?).

    also, does anyone know the easiest way to pull the carbs on this model? i
    suspect that i'll need to remove the airbox and i might need to take out
    the battery box to get the airbox out.

    thanks, all.

    Sean J Kelly, Aug 22, 2005
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  2. Sean J Kelly

    NA Guest

    The carbs need a thorough cleaning... search this newsgroup, there
    are plenty of practical advice written up by 'krusty kritter'.
    The float valves are probably sticking and not sliding properly.
    vacuum lines... have your friend get the *2* service manuals for
    the EX250F. You can also view the parts diagrams on Kawasaki's
    The carbs are easy and straightforward to remove. Not necessary
    to remove airbox and battery box. The rubber boots are a bit
    tricky to remove and reinstall with limited spacing but no major
    problem. Good luck.
    NA, Aug 22, 2005
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  3. Well, your idle jets are all plugged up, and setting your idle RPM that
    high just defeats the starting enrichener and makes your engine even
    harder to start. You probably have runaway idle RPM when the engine
    starts, too, because the throttle butterflies are open so far the
    transition ports are exposed.

    This type of constant vacuum carburetor probably doesn't have a real
    "choke" like a car's carb. Instead, it has a starting enrichener
    device, which is a tiny carburetor built into a bypass passage that
    goes around the starter butterfly. The enrichener has it's own starter
    jet in the float bowl, and when you turn the "choke" lever to full ON,
    you're opening the enrichener valve and letting air go AROUND the
    butterflies, which need to be CLOSED.

    IOW, don't twist the throttle grip when trying to start the engine.
    It's supposed to start and idle at a reasonable speed when the "choke"
    lever is in the ON position. The starting enrichener won't work if you
    twist the throttle or if you set the idle RPM too high. When the engine
    resists starting, what you want to do is turn the master IDLE knob
    COUNTERCLOCKWISE, to get the throttle butterflies CLOSED all the way.

    If the engine still resists starting, the starter jet and idle jets are
    all plugged up. Cleaning out idle jets has been described a thousand
    times in this newsgroup, just like the starting enrichener, stuck float
    valves, runaway idle RPM, etc.
    Well, when you leave a motorbike sit for a long time, the rubber seal
    in the petcock sticks to the selector lever, then when you turn the
    lever to the PRI position to start the engine, a little bit of rubber
    between the four holes in the seal rips out and then that seal leaks.
    If you have a vacuum petcock, the
    automatic shutoff valve can leak too, if there is crud in the petcock
    That type of carburetor has an o-ring seal. If the motorbike has set
    for long enough that the gasoline evaporates, the o-ring dries out and
    shrinks. When you turn the petcock to PRI to fill the float bowls,
    gasoline will weep past the o-ring until it swells up again and stops
    the leak.

    If you're seeing gasoline run in a steady stream out the overflow hose
    when the engine isn't running, that tells you that (1) the shutoff
    valve in the petcock is leaking, and (2) the float valve in the
    carburetor is leaking or the float is stuck or set too high.

    Float valves can get stuck from gum and varnish accumulating on the
    edges. The float valve is usually square or triangular to guide ut up
    and down in the float valve seat, but it doesn't take much crud to make
    it stick.

    Alternatively, a tiny bit of crud on the float valve seat itself will
    cause the float valve to be unable to seal and gasoline will still leak
    past when the float has risen the correct shutoff position. I found a
    tiny bit of crud using a magnifying glass and removed it so my float
    valve would seal.
    That sounds like your vacuum operated petcock is OK then. Concentrate
    on the
    If you can't clean the plugs with a tiny bit of sandpaper rolled up so
    it gets in between the metal shell and the porcelin insulator, replace
    the spark plugs. They are about $2.50 USD each.
    Constant vacuum carburetors have a throttle butterfly like a car's
    carburetor instead of a slide that is directly actuated by a cable,
    like on older motorcycles from the 1960's and 1970's.

    There is a big rubber vacuum diaphragm under each of the round caps on
    top of the carbs. When the rider turns the throttle grip, the
    butteflies open and airflow increases through the carburetor.

    Air is sucked out from on top of the diaphragm. Air coming in from
    underneath the diaphraghm lifts a plastic slide which controls the
    airflow through the carb, keeping the vacuum in the venturi area
    constant. The diaphragm and slide take direct control of the fuel air
    mixture out of the rider's wrist.

    There is a tapered needle attached to the slide. It's called the "jet
    needle". It goes in and out of a hole in a brass tube called the
    "needle jet". This motion controls fuel/air mixture when the slide is
    1/4 to 3/4 of the way open. The slide motion has no direct relationship
    to the amount you twist the throttle. If you twist the throttle open
    too quickly, the vacuum slide may not open, it may actually close
    instead, to keep you from polluting the atmosphere with excess unburned

    Once the slide has lifted all the way, the restriction of the jet
    needle in the needle jet tube is less than the restriction of the hole
    in the main jet. At this point, the fuel air mixture is controlled by
    the main jet.

    But most riders do not understand that they are rarely running on the
    main jet. They fool around with main jets, thinking they are somehow
    "tuning" their engine for performance, because they heard some racers
    talking about main jets. The engine may or may not be running on the
    main jet when the throttle is wide open, the vacuum slide determines
    where the jet needle taper is in relation to the needle jet hole.

    Most of the time, your engine is running on the idle jets, or on the
    jet needle taper. That's why it's important to keep you idle jets clean
    by running a few ounces of fuel system cleaner containing xylene and
    alcohol through a full tank of gas on a regular basis. Don't bother
    with fuel system cleaners containing petroleum distillate. Those
    cleaners are for fuel injectors. They lubricate the pintle valve in the
    injector, making it slide freely.
    The latter hose is the vacuum hose that pulls on a little rubber
    diaphragm and that opens the automatic fuel shutoff valve in the
    petcock. The other hoses are float bowl vent hoses. It's necessary to
    vent the float bowl, because it's outside *air pressure*, not vacuum,
    that causes fuel to flow through the jets into the carburetor venturi.
    I've found that some of the rubber hoses between the carb and the air
    box will slide back into the air box so I don't have to remove it.
    Older hardened rubber hoses can be softened by spraying with aerosol
    carb cleaner containing xylene. Then when the xylene evaporates, the
    hose returns to the previous condition. Keep aerosol carb cleaners off
    your paint though.
    krusty kritter, Aug 22, 2005
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