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General carbies

Discussion in 'Technical and Troubleshooting Torque' started by ibast, Feb 14, 2005.

  1. Ok a question about Carbies.

    Main jets are used to limit fuel flow (top end).
    The needle is to regulate flow.

    A number of questions arise:

    a) Is the needle jet the jet through which the needle slides?
    If so why would you vary the size, when you normally vary the shape and setting of the needle?

    b) Is the pilot jet just at idle or at low speed also?
    At idle, the mixture is effected by the mixture screw, so what is the correlation?
    Does the mixture screw set overall mixture?

    c) What is the air jet?
    Is this for the choke?
  2. Yes
    The shape of the needle regulates the air:fuel ratio over the needle's range of influence (~ 15% to 35% of throttle opening). The jet regulates ratio from about 25% to 85% of throttle opening (YMMV quite literally- depends on the carb). These overlap but the point at which each has the greatest influence is about 25% throttle for the needle and 60% for the jet.
    Take a look at http://www.keihin-us.com/tune1.htm
    Changes to the needle jet have an effect over the full range of the needle jet's influence, but their effect effect is more pronounced at larger throttle openings. Altering the diameter of the needle allows you to fine tune the lower end of the needle's influence independently of the needle's upper range effect.

    Whether you change needles or jets also depends on the carb - I'm told that with Mikuni carbs you change the needle, with Keihins change the jet. I'm fairly familiar with the innards of my Mikuni but I don't have the endurance to attack the 4 Keihins on my other bike.

    So, if your bike runs lean at 60% throttle, you'd be looking to increase the bore of the needle jet. Of course, if it also runs lean at 50% and 75%, it might be necessary to alter the length of the needle and bore of the main jet and leave the needle jet alone. Having upped the needle jet, it might run rich at 50%, so its time to reset the needle length. As you can see, setting carbies is a black art at the best of times - altering the taper of the needle (and a needle may have a range of tapers over its length) is beyond my comprehension.
    The pilot jet operates throughout the throttle range, but its influence diminishes dramatically after 25%
    The effect of altering the screw setting depends on whether your carb has an air mixture screw (which varies the amount of air) of a fuel mixture screw (which alters the amount of fuel). I'm guessing that you have an air screw, so turning it in enriches the mixture.
    No - only the mixture supplied by the pilot jet.
    The air jet controls the amount of air being fed to the idle jet. The air screw opens or closes the air jet, regulating the idle mixture
    No. The choke serves to enrich the mixture. This can be done by closing off the air jet, but this is an additional mechanism that influences the air jet, not a function of the air jet.
  3. a) The jet needle moves in and out of the needle jet. The jet needle usually hangs of the bottom of the slide, and thus moves in and out as the slide is adjusted. Both the jet needle and the needle jet can be changed, and whether you adjust the needle jet or the jet needle would depend upon desired outcome for how much extra fuel you needed to supply the carby throat. If you vary the jet needle size, then you are changing the timing of the level of fuel that enters the throat. If you change the jet needle shape/taper, then you are changing the quantity of fuel that enters the throat. You may wish to change both the size and shape to effect the amount and timing of fuel entry. Changing the needle jet generally achieves the same result as changing the jet needle shape (allow more or less fuel into the throat), and is generally used to achieve a larger flow rate of fuel than would be achieved by just adjusting the jet needle taper.

    Note: For those reading thus far and confused, think of a hole with an arrow (or bullet stuck) in it. Bullet/arrow = jet needle, and hole = needle jet. If the bullet/arrow is in the hole, then no fuel can pass through the hole. Slowly extract the bullet/arrow and fuel can then flow through the hole. Change the size of the hole to affect fuel flow. Also change the length of the bullet/arrow and the angle of the point of the bullet/arrow and you also change when and how much fuel goes through the hole.

    b) Pilot jet works at slow speed (or more accurately throttle usage, because you can be using the pilot jet at high speed depending upon which gear and how much throttle you have on.) Pilot jet generally works from no throttle to 1/8th of throttle travel, after which the needle jet kicks in and it's affect overrules/overpowers the effect of the pilot jet. (Main jet kicks in at about 3/4th throttle travel and overpowers/overrules needle jet). The mixture screw is controlling the fuel flow into the throat at idle time with the amount of air being sucked in by vacuum, thereby setting the revs at idle. Beyond idle it is the throttle controlling fuel delivery volume.

    c) Used to 'bleed' a small amount of air into the throat when the butterfly flap is fully closed. As you say, generally for cold starts with the choke on.
  4. Thanks guys.

    I actually found this site not long after I posted yesterday.


    I understand much more now.

    the general impression I get is:

    The needle regulates the rate of change (i.e. slope of curve), whereas the needle jet size regulates the overall fueling (i.e. the y intersect), in the needle range.

    So if fueling is constant but rich or lean you change the jet. If fueling is not constant. e.g. lean up high but rich down low then you change the taper of the needle.

    You change the pilot jet when the screw is neer its limit. I gather this is to ensure good throttle responce just off idle.

    I'm still not sure why you would change both an air jet and a pilot jet.

    Do most carbies have both? Or is that what I am missing?
  5. Thanks Androo,

    that filled in a few more gaps.

    I'm a bit sceptical about he plug chop thing however.

    What happens if I now go mucking around with different temp plugs? I could end up with what looks like a rich carbie, but is in fact too cold a plug. I would then need to lean the mixture and the result would be too lean and thus dangerous.

    Add mcuking around with ignition advance, which changes plug colour, running temperature, idle speed, off idle performance, etc, into the mix and you could spend months chasing your tail.
  6. haha yeh, I think that 'plug chop' method is a bit outdated. reading spark-plugs is a bit iffy anyway - better to just get the bike on a dyno with exhaust gas analysis.

    as for tuning, you'd commonly pick a brand of jet kit (i.e. forget about needle tapers etc) and tune with:
    -pilot screw
    -jet needle clip
    -main jets

    if you want some food for thought, mototuneusa.com has interesting/controversial articles