cvk34 vulcan 500 carb problem

Discussion in 'Motorbike Technical Discussion' started by matt.trenholm, Nov 4, 2006.

  1. Okay, I've had these carbs apart a million times. I've synced them,
    I've cleaned them, I've done everything I can think of. Bike worked
    fine before I hauled it off for a rejet. Bike is a 1997 Vulcan 500 with
    CVK34 carbs.

    But now I can't even get the bike to run with all original equipment.
    I'm at wits end. The bike starts and idles, but when I give it any
    throttle it dies out, like its either being starved or being given too
    much gas.

    Please help!
    matt.trenholm, Nov 4, 2006
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  2. matt.trenholm

    B-12 Guest

    Lean idle mixture could be due to low fuel level in the float bowls,
    stuck float valves, plugged up idle passages and jets, or the idle
    mixture screws might be set too far closed.

    Have you drilled out the EPA anti-tamper plugs yet?
    B-12, Nov 4, 2006
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  3. Yes, the pilot screws were about 2 turns out when I disassembled the
    carbs. They're about 1.5-2 turns now, give or take a quarter turn.
    matt.trenholm, Nov 4, 2006
  4. matt.trenholm

    B-12 Guest

    What size idle jets are in the carbs now? That's the key to
    understanding why your information about the number of turns out is

    Since the orifice hole is based upon percentage of a millimeter ( # 30
    idle jets having a hole diameter that is 0.3 mm in diameter ), and the
    area of the idle jet varies exponentially as to the square of the
    radius of the hole time pi, NOBODY can tell you how many turns out the
    idle mixture screws should be, you have to find that by

    Like, if you work the area formula for a # 45 idle jet vs. a # 30 idle
    jet, you'll probably find that the # 45 idle jet passes twice the fuel
    that the # 30 jet passes.

    Are the floats installed right side up? And when you set the floats
    with the carbs on the bench upside down, did you measure from the
    bottom of the float (which is the top of the float when it's upside
    down) to the metal gasket surface on the carburetor body itself?

    When you read the specifications on setting the float level, you might
    see a number like
    14 mm, plus or minus 1 mm. In this case, since you set the floats with
    the carbs upside down, a 13 mm setting is a HIGH float level, making it
    easier for the carb to suck fuel out of the float bowls, and 15 mm is a
    LOW float level, shutting the fuel flow off earlier and making it
    harder for the carb to suck gas out of the bowl.

    Do the float valves move freely when you turn the carbs over?
    B-12, Nov 4, 2006
  5. matt.trenholm

    GaZ Guest

    Is that the type with diaphragms? (think so) check diaphragms and the
    fitting of the slide to diaphragm. If this is loose the slide will not rise
    with the diaphragm
    GaZ, Nov 4, 2006
  6. matt.trenholm

    B-12 Guest

    Bzzzzt! Wrong answer but thanks for playing "I Don't Understand How CV
    Carbs Work".

    The air is pumped out of the diaphragm chamber through a small hole (or
    two small holes) drilled in the slide itself, so loose fitting of the
    slide won't affect acceleration from idle.

    The slide doesn't need to rise at all for the carburetor to respond to
    small throttle openings. The throttle butterfly covers up two of a
    triangular pattern of transition ports and partly covers the third
    transition port.

    When the throttle butterfly moves off the idle setting, it uncovers the
    transition ports, allowing sufficient fuel to accelerate the engine
    from idle speed.

    Only then can the engine pump the air out of the chamber above the
    diaphragm, allowing air pressure from underneath the diaphragm to lift
    the slide.

    If the diaphragm has a large tear, the pressures above and below the
    diaphragm will be equalized and the slide won't lift. That's as close
    as I can get to your theory about a loose fitting slide causing the
    B-12, Nov 4, 2006
  7. matt.trenholm

    oldgeezer Guest

    The obvious is checking the carbs, but
    try to recall what else you did besides working on the carbs.
    You could have made an error somewhere else.

    Talking about the carbs: what about the connection carb-cylinder.
    oldgeezer, Nov 4, 2006
  8. matt.trenholm

    oldgeezer Guest

    B-12 schreef:
    Erm.. I read that GaZ ment a loose diaphragm, no longer connected
    to the slide, he did not mean too much play beween slide and housing.

    oldgeezer, Nov 4, 2006
  9. matt.trenholm

    Manjo Guest

    I read on another NG that somene had taken their CV carbs apart and had
    not installed the needles correctly. As a reuslt, the needle was not
    being lifted as the throttle was turned. If you don't have a shop
    manual, go to and follow the links (many) to the
    owner info/parts diagram/motorcycle, year, cc's, etc. for your
    year/mkae model. The exploded carburetor parts diagram will show
    pretty accurately how the needle and parts fit into the diagram. Good

    Manjo, Nov 5, 2006
  10. matt.trenholm

    B-12 Guest

    Bzzzt! Wrong answer, but thanks for playing, "I Don't Have A Clue About
    CV Carbs".

    The symptom is that the engine starts and idles, but dies when given
    any throttle.

    This indicates that the transition ports are blocked by crud.

    It doesn't matter whether the slides rise by being pulled up by the
    diaphragms. The needles are straight for almost an inch, no taper at
    all, so the orifice area between the needle jet and the jet needle
    doesn't change, no extra fuel is admitted to the mixing chamber.

    I rode a single-cylinder motorcycle for 20 miles once, with the slide
    stuck closed by gum and varnish.

    Nevertheless, the engine developed enough power to run on the idle
    circuit, and I climbed up a 500 foot grade three times, wondering why
    the throttle response was so poor.

    The motorcycle ran a lot better when I sprayed some B-12 onto the
    slide, freeing it.
    B-12, Nov 5, 2006
  11. Hey B-12,

    The idle jets (and everything for that matter) that are in the carb now
    are what was in it, #35's. I havent touched the floats, but the fluid
    level is about 5mm short of the base. While this isn't optimum I don't
    imagine it would be bogging the bike down.

    I've had the carbs apart a million times to try to remedy this but it
    could be possible the transition ports are still plugged. I've tried a
    toothbrush, I've tried a twist tie, carb cleaner and brake cleaner,
    compressed air, but I dunno what else to do with it.

    Although, I did read a similar post on an HD forum about someone using
    a CVK40, he put the needle jet holder in the wrong way so the needle
    couldn't seat right and it was causing the bike to bog because of too
    much fuel. I had a similar problem in that it was running way too rich
    and that was because I had installled the needle into the spring seat
    instead of needle first, spring seat second. So to whoever recommended
    looking closely at the parts diagrams, thank you, it DOES help the
    novice :)
    matt.trenholm, Nov 5, 2006
  12. oldgeezer wrote:

    The only other thing I did to it was fix the ignition wire boot. I had
    replaced the spark plugs a while ago and the brass connector in the
    ignition wire boot had come undone, leaving me running on one cylinder
    till I got it home. I fixed the boot (I believe the order went spring -
    resistor - brass fitting) and while I was there I hauled off the carb
    because I was looking for an excuse to, I had bought a rejet kit the
    week before.

    I dont think it would be a problem with the ignition wire, unless I put
    the resistor in the wrong way. But if I did wouldn't it be giving off a
    weak spark, or no spark at all on that cylinder? I've tried pulling the
    plug on one cylinder at a time to see if one specific carb was causing
    the problem, because my bike ran very well on one cylinder, but both
    seem to be affected.

    Other then that, thats all I had done to it.
    matt.trenholm, Nov 5, 2006
  13. matt.trenholm

    B-12 Guest

    Since the tolerance for setting floats is plus or minus ONE millimeter,
    a fuel level that is
    FIVE millimeters too low would make the venturi's job of lifting fuel
    out of the float bowl

    Are you sure that the floats are installed the right way up? When you
    remove them, look for a tiny witness mark on the tang that operates the
    float valve. If you see the tiny witness mark on both sides, you may
    have had the flats installed upside dow, shutting off the gasoline

    You need some sort of depth gauge to accurately check float height. You
    can buy a cheap plastic caliper if you don't already have a good
    quality machinist's caliper made of metal.
    I understand your that your complaint is that you start the engine and
    it idles just fine and warms up, but when you open the throttle, the
    engine dies. That's NOT bogging.

    Most riders don't realize that there's a difference between bogging
    down and an engine that's starving for fuel air mixture. It's almost
    impossible for a CV carburetor that's set up correctly to raise the
    slide enough so the engine will bog because the rider opened the
    throttle too much.

    Riders were able to do this with the older style slide valve
    carburetors because the slide was pulled open directly by the throttle
    cable. The rider would give the engine too much throttle and it would
    suck in a huge gulp of gasoline that it couldn't burn and that unburnt
    gasoline would be ejected out the exhaust pipe.

    The CV carburetor takes direct control of the slide away from the
    rider, and it won't allow a large flow of gasoline into the mixing
    chamber if there isn't a corresponding large flow of air as sensed by
    the diaphragm.

    Ultrasonic cleaning might be necessary if you can't get carburetor
    cleaner to squirt out of all three transition ports and the idle
    mixture port. The idle mixture system gets air through the pilot air
    jet (which may be in the carburetor inlet bell, or under the diaphragm
    cap). The idle mixture system gets gasoline from the the idle jet in
    the float bowl.

    The pilot air and the idle gasoline mix, and then the passages split to
    go out the transition ports and the idle mixture port.

    If you have the carbs off the engine, block the throttle butterflies
    closed, cover the idle mixture port with one finger and the pilot air
    inlet with another finger, and squirt carburetor cleaner through the
    idle jet. The carb cleaner must squirt out of the three transition
    ports. If it doesn't, they are plugged up.
    If you install the spring seat and then the needle, the carburetor will
    come onto the main jet too early and the mixture will be far too rich
    for the amount of air passing through the carb. The engine will blubber
    and fire every other stroke. This is called "8-stroking".
    B-12, Nov 5, 2006
  14. matt.trenholm

    B-12 Guest

    I should have written:

    "If you have the carbs off the engine, block the throttle butterflies
    B-12, Nov 5, 2006
  15. matt.trenholm

    oldgeezer Guest

    So it looks like the carburettor after all.
    Since the bike ran (until now), I doubt clogging. Furthermore, you have
    two carburettors (I think). If one was clogged, the bike would pick up
    at least on one cilinder.

    Suppose it is a 'common' (both cilinders) problem:
    What about fuel cap vent? Start without a cap on the fuel
    tank and try the engine. (Don't smoke).
    Is there any common fuel filter dirty (petcock)?
    Vacuum hose to petcock okay?
    Air filters? (They both get dirty at the same time).

    Another common thing: you may have assembled both carbs
    incorrect. Here are some things I can think of:
    Have a look at the top end (diaphragm). The spring seat has kinda
    5 fingers. The spring seat can be rotated. One of the fingers may
    block the hole at the bottom of the vacuum piston. You could
    have blocked that hole. Remedy, rotate the spring seat .

    The needle jet goes into the body with the smaller
    diameter first (carburettor upside down). Could be you did that wrong.

    Fuel level can be measured as follows: You need the petcock be
    open (has this bike a PRI position on the petcock? If not,
    you gotta suck the vacuum line).

    Take a piece of clear plastic tube that will fit on the carb drain
    Set the bike so that carbs ar horizontal (need a jack for that).
    Hold the tube alongside the carb body pointing up.
    Open petcock (PRI or suck vacuum).
    Open drain carb screw. Wait until fuel level settles in
    tube. Do not lower and raise the tube, just keep it steady.
    If you wiggled the tube, close drain screw, empty the tube
    and retry.
    As far as I know, fuel level must be 0.5 +- 1 mm above the
    bottom edge of the carb body (the lowest split_casing line
    of the carb).

    oldgeezer, Nov 6, 2006
  16. matt.trenholm

    GaZ Guest

    Yip, thats what happened to mine.

    ( air is pumped out diaphragm chamber ? )
    GaZ, Nov 6, 2006
  17. matt.trenholm

    B-12 Guest

    I suppose that problem could be cured by silicone rubber cement.
    Plug the hole(s) in the slide and try to raise it with your fingertip.

    The Dynojet kits and the Factory Pro kits usually include a drill for
    enlargening the holes so the slide will lift quicker as air is pumped
    out from above the rubber diaphragm.

    Enlargening the holes too much will cause the slides to fly up too
    quickly on acceleration and drop too rapidly on deceleration. Throttle
    control suffers...
    B-12, Nov 7, 2006
  18. matt.trenholm

    GaZ Guest

    As you are naturally careful in handling the slide assembly, it is not easy
    to spot that they are pulling apart as the return spring 're assembles' them
    before they are removed for inspection. I lightly centre popped them and it
    seems to work fine.

    The slide rising is a product of depression bled through from the intake,
    but I was being petty, I wish I had not posted it. The idea here is to try
    to assist not score points, I apologise.
    GaZ, Nov 7, 2006
  19. matt.trenholm

    Manjo Guest


    Is there any chance you reversed the main and pilot jets when you put
    the carbs back together? I vaguely remember the jets have the same
    outside diameter at the base, but I'm not sure about the threads. Just
    a thought.

    Manjo, Nov 7, 2006
  20. matt.trenholm

    B-12 Guest

    Vacuum cannot lift anything, the slide is not sucked upwards by vacuum.

    Engine vacuum sucks most of the air out of the diaphragm chamber
    through the hole(s)
    in the slide.

    Air at near-ambient pressure enters the oval hole in the intake bell of
    the carb, and lifts the slide, overcoming light spring pressure and the
    friction and weight of the slide.

    Depending on the condition of the air filter element and the fact that
    the intake air is in motion, this pressure may be around 0.25 pounds
    less than ambient pressure.
    B-12, Nov 7, 2006
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