Choppy throttle response around 4000 RPM

Discussion in 'Motorcycle Technical Discussion' started by Masospaghetti, Oct 22, 2005.

  1. Hey all -

    1980 KZ750 LTD back again.

    When I open up the throttle on the freeway going about 50 mph, the power
    delivery is not smooth at all. It's smooth if im going around 65, and I
    haven't noticed it at lower speeds either. What could this be a symptom of?
     
    Masospaghetti, Oct 22, 2005
    #1
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  2. Masospaghetti

    OH- Guest

    Weeell, it COULD be a symptom of this bike not delivering
    smooth power in whatever gear you're in, at 4000 rpm.

    Honestly.

    The cure might be to change down one gear to avoid the
    symptom.

    I assume your carburettors are synchronized and your
    spark plugs reasonably fresh.

    SWHAC will be along shortly.
     
    OH-, Oct 22, 2005
    #2
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  3. Yes, it doesn't depend on the gear i'm in. However, I get a lot of
    vibrations at higher RPM so I try to avoid them if at all possible.

    Spark plugs are new, carbs are synch'ed.
     
    Masospaghetti, Oct 22, 2005
    #3
  4. Masospaghetti

    fweddybear Guest

    Try rejetting......if your plugs are burning on the brownish side, thats
    a good sign..... if a little white, then its running a lil lean.... if
    black, too rich.... hope this helps..

    Fwed
     
    fweddybear, Oct 23, 2005
    #4
  5. Weren't you having some problems with low compression on one cylinder
    and a head gasket leak?
     
    krusty kritter, Oct 23, 2005
    #5
  6. You might try shimming the needles. Though, before you do that, having the
    bike run on a dyno with exhaust gas analysis might be useful.
     
    Michael J. Freeman, Oct 23, 2005
    #6
  7. I thought so, I must have done the test wrong though, because now the
    compression is very good on both cylinders - 150 psi cold.

    The bike runs well now overall. Idles around 900 rpm, no backfiring,
    plugs aren't AS sooty as before (little bit of soot around the rim of
    the plug, the electrode is still clean though). Filled the handlebars
    with birdshot and it's much more comfortable.

    The head gasket may have stopped leaking too - not sure how thats
    possible - but its a very slow leak now. Most of the oil was coming from
    the chain tensioner with a bad seal.
     
    Masospaghetti, Oct 23, 2005
    #7
  8. Vetical twin do send a lot of vibration up towards the handlebars, but
    that model has a counterbalancer. Have you done anything to adjust the
    counterbalancer drive chain? It could be that it's loose.
    If it's not the counterbalancer adjustment maybe the bike is lean
    surging?
    When an engine lean surges, it pulls hard for a second, then it stops
    pulling, and starts pulling again for second, etc.

    If that's what it's doing, you might try turning the fuel/air mixture
    screws out half a turn, or shimming the needles a little higher as
    another poster suggested.

    You can get a few 1/2 mm X 3 mm washers at a hardware store for about
    $0.49 if you buy them in the package.

    If you make the mixture too rich, the engine will start "8-stroking".
    This makes the engine sound a bit like a 2-stroke that's all loaded up
    with oil and is fouling its plug. It's a "Br-r-r-r-r!" sound.

    The engine will fire every other compression stroke and the exhaust
    will sound blubbery and you'll notice a lot of vibration when you try
    to speed up.
    The engine is expending half of its effort trying to pump unburn
    fuel/air mixture when it 8-strokes.

    Of course the spark plugs will carbon up if the engine is 8-stroking.
     
    krusty kritter, Oct 23, 2005
    #8
  9. No, unfortunately...but from what I understand, adjusting the balancer
    chain requires tearing down the entire engine :(
    Hmmm...hope its not the latter...I've had a hell of a time trying to
    lean this thing out.
     
    Masospaghetti, Oct 24, 2005
    #9
  10. Masospaghetti

    Matt Guest

    How are your air cleaners?
     
    Matt, Oct 25, 2005
    #10
  11. It's the stock airbox, the filter is about 2 months old. It looks clean
    and smells slightly of gasoline.
     
    Masospaghetti, Oct 25, 2005
    #11
  12. If it smooths out when you crank the throttle on, might be a touch
    too rich. (My experience with CV carbs anyway).
     
    Rob Kleinschmidt, Oct 25, 2005
    #12
  13. Masospaghetti wrote:

    Have you tried adjusting the idle mixture screws at all since you
    installed the new ignition points? There is the possibility that the
    idle mixture screw is actually a pilot air screw that works backwards
    from the normal pilot gas screw that CV carbs usually have.

    You might try adjusting both screws to 1.5 turns out and see what
    happens with that setting, whether it's really rich or if it leans the
    mixture up.

    You'd know if the mixture was getting richer by the fact the the idle
    RPM would hang up when you blipped the throttle open and closed.
     
    krusty kritter, Oct 25, 2005
    #13
  14. What do you mean by "crank the throttle"? It get progressively more
    obvious the more I open it up.
     
    Masospaghetti, Oct 25, 2005
    #14
  15. Masospaghetti

    Matt Guest

    Also check the tachometer-cable fitting. There may be a leak that you
    can fix by tightening a nut there. Take a look while it is idling.
     
    Matt, Oct 26, 2005
    #15
  16. Missed this or I would have responded sooner. I've found a too-rich
    condition can show up as uneven running at a steady throttle that'll
    get better when you open it up. If it gets worse when you open it up,
    I'd bet on too lean.
     
    Rob Kleinschmidt, Oct 28, 2005
    #16
  17. I'll try that.

    I made sure the pilot jets were the right size, they were...and valve
    clearance is fine as well. It's better than it was though, i'm averaging
    about 30 mpg instead of around 20.
     
    Masospaghetti, Nov 1, 2005
    #17
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