Also have trouble 1983 Shadow VT750

Discussion in 'Motorbike Technical Discussion' started by red, Jul 25, 2005.

  1. red

    red Guest

    Bike was sitting for 5 years. Starts and idles well. I can accelerate
    up to
    3000 rpms and no more. Starts popping and boggs down but only above
    3000 rpms. Bike has after market mufflers--tried to restrict (like
    stock mufflers) but did not help problem. Carbs rebuilt 4
    times--clean. Have syncronized carbs--no improvement. Any help would
    be appreciated.
    red, Jul 25, 2005
    1. Advertisements

  2. 1. Plugged idle jet
    2. Plugged idle passage
    3. Stuck vacuum slide
    4. Torn or punctured diaphragm
    5. Stuck float valve
    6. Float level too low
    7. Plugged air jet
    8. Loose electrical connections in ignition system
    9. Plugged up air filter
    10. Vapor trap in fuel line to pump
    11. Clogged fuel filters in tank or carbs
    12. Induction system air leak
    krusty kritter, Jul 25, 2005
    1. Advertisements

  3. Also, add intake valve deposits prevent intake valve from seating. This
    causes spitting back out the carburetor...
    krusty kritter, Jul 25, 2005
  4. red

    t Guest

    where are you located?
    t, Jul 25, 2005
  5. Most of the time this is fuel relted problem. Want verification? Whle it
    is bogged down, spray a little carb cleaner into the intake. Yamaha makes
    a liquid carb cleaner that works darned good.

    When you rebuilt the carbs did you spray your Berryman's through each
    orifice? To be honest it sounds like your main fuel circuit is clogged.
    Read the appropriate manual and figure out the main fuel circuit and make
    sure it is free of any occlusion.

    R. Pierce Butler, Jul 26, 2005
  6. red

    red Guest

    Question?: Why would plugged idle jet affect rpms above 3000 (#1 and
    #2). Bike runs well enough below 3000 but cuts out above. I have the
    air cleaner removed and vaccum slide/diaphams working well (#3 and
    #4). Air cleaner not a factor as it is not connected to test (#9).
    Unhook fuel line and rebuilt electric fuel pump--working well (#10 and
    #11) No induction system on ’83 (started in ’84) I like your idea
    about intake valve deposits--any idea how to remove without teardown?
    Bike has hydraulic lifters--possible sticky lifter?? Many thanks for
    input--bike is getting frustrating to problem solve and it helps to
    bounce ideas off you guys.
    red, Jul 26, 2005
  7. red

    red Guest

    Central Minnesota
    red, Jul 26, 2005
  8. red wrote:

    When you're cruising down the highway at 65mph and your throttle grip
    is only turned 1/8th of a turn, the throttle butterflies are hardly
    open. CV carbs have HUGE idle jets, compared to the idle jets on the
    older slide valve carbs.

    As I recall, the VT750 had about #48 idle jets. That's a jet with a
    hole in it that's 0.48 millimeters in diameter. By contrast, my
    GSXR-750 has #17.5 idle jets in its 33mm smoothbore racing carburetors!

    So engine vacuum is high at small throttle openings and the engine can
    be running off the idle jets any time the load on the engine is light.
    The engine is running off of fuel air mixture coming from the idle jet
    and idle passage at idle, and the flow of fuel through the idle
    passages drops off until nothing is coming through the idle passages at
    wide open throttle.

    The idle jets and idle passages had better be squeaky clean to pass
    enough fuel to the engine.

    As the rider opens the throttle more and more, vacuum lifts the
    slides and the jet needles come up out of the needle jets, allowing
    more and more fuel as more power is needed. The hole in the needle jet
    is HUGE, about 1/8th of an inch in diameter. It's almost impossible to
    plug up the fuel hole in the needle jet, but the little air emulsion
    holes in the sides of bleed type emulsion jets can get plugged up with
    gum and varnish.

    Your engine isn't running on the main jets until the throttles are
    3/4th's to full open, and then only when engine vacuum is high enough
    to lift the slides all the way. Badly plugged up main jets can begin to
    affect engine performance from 1/8th throttle to full throttle, but the
    way CV carbs depend on the idle jets, I would bet that you could ride
    for hours on the freeway and still be running on the idle jets if you
    never twisted the throttle past 1/4th of a turn...
    Run about 3 ounces of Chevron's Techron concentrate through the
    gasoline for four tanks of gas.
    krusty kritter, Jul 27, 2005
  9. red

    t Guest


    mine has been frustrating as well, seemed to be running too rich (after
    much consulting with Mr. Kritter), and backed down on the pilot screws.
    but i have the same problem you do with it cutting out at higher RPMs.
    i cleaned the carbs, checked the fuel pump, slides, etc. still don't
    have it quite figured, yet.
    t, Jul 28, 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.