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True purpose of the Mixture Screw?

Discussion in 'XJ Technical Chat' started by MrSeca, Dec 19, 2020.

  1. MrSeca

    MrSeca Active Member

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    What is the Mixture Screw called?
    The Idle Jet Screw or
    The Pilot Jet Air Screw?

    Many think the Mixture Screw is for the Idle Circuit only.
    So next Question is:
    Is the mixture screw for the Idle Circuit Only OR does it affect
    both the Idle circuit AND the Secondary circuit?
    And what separates the two circuits?

    So for example, if you're running rich do you adjust the Mixture screw or is there something else you would have to do to fix that?
    And what separates the two circuits?
     
  2. FJ111200

    FJ111200 Active Member

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  3. Minimutly

    Minimutly Well-Known Member

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    Except that it shows mixture control by pilot air screw. On our carbs it isn't. On both mikunis and hitachis the mixture screw adjusts the fuel which bypasses the carburettor, controling idle and overrun mixture. Sure, it adds fiel at all times, but the ration of this amount to the overall fuel delivered with the throttle open is tiny.
    Se effectively, the mixture screw is for idle only, and should be treated as such - in equals less fuel, out equals more.
    Hope this helps?
     
  4. XJ550H

    XJ550H Well-Known Member Premium Member

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    this link leads to cutaways of the carbs showing how the passages connect up
    Something New, Something Naked

    this will help
    THE SECRET LIFE OF CARBURETORS

    the mixture screw tuns the pilot jet circuit to a proper airmixture/fuel flowthrough the little holes in the carb throat, helping adjust to epa requirements for emmisions.
    it also sets the circuit so you are not running to lean or rich. for the pilot circuit.

    your motor runs off of the pilot jet circuit untill 2 to 3 k rpm where the mains start to kick in.

    when rejetting the mains are configured first then the pilots while the pilot does add fuel constantly it does not do much if anything to adjust main jets rich/lean issues

    it is also part of the carb sync to get the motor balanced . as it helps adjust the fine tune of ideling, once mixture is dialed in the sync can be done . then you readjust mixtures and resync as needed.

    the pilot jet circuiit has an air jet and a fuel jet and mixture screw. .
    lppk at the photos air comes down the air jet mixing with the fuel flow from the pilot jet.
    in the top of the carb throat you will see little holes, at the butterfly s the holes dump fuel mixture into the throat then there is the other hole closest to the motor where the tip of the mixture protrudes.
    when you addjust the screw it controls the amount of fuel mixture flow through the little holes to fine tune the amount of asperated fuel being used.
    the mixture also flows out this hole



     
    Last edited: Dec 20, 2020
  5. tj.

    tj. Active Member

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    In the Yamaha service manual it is referred to as the "pilot screw".
     
  6. Minimutly

    Minimutly Well-Known Member

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    I will just say that we all should have a duty to not confuse where we are not sure. The above answer has a shortcoming - Quite a major one.
    I won't say where it is @XJ550H, perhaps someone else can be bothered.
     
  7. XJ550H

    XJ550H Well-Known Member Premium Member

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    main and pilot jet circuits are not connected to each other .

    the output of fuel from both come together in the throat of the carb.
     
  8. Minimutly

    Minimutly Well-Known Member

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    Ok, I'll put you out of your "misery". The assumption you made that the mixture screw can adjust the fuel coming out of the progression holes is wrong - they are in the line feeding the screw, but the outlet of the screw is engine side of the plate and totally independant, unless you were to open it fully and maybe actually rob the progression circuit a little?

    I also have issues with @k-moe and @chacal s assertion that the progression holes are there for (or at least also) to provide, snap enrichment fuel. I haven't got around to looking for the proof that they can't or don't yet, so am leaving that one lie.
    OR, perhaps I will just say this now - if the idle jet is sized for idle and progression under steady state, how can it then provide the numbers of times extra fuel needed for accell fuelling?
     
  9. Minimutly

    Minimutly Well-Known Member

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    Ok, without resorting to looking up my fuel injection data, which might or might not tell me anything conclusive, I looked up my Dellorto jetting charts. In each case the accell fuel jet is at least 2/3 the size of the idle jet. That is, for a given vacuum it would give 2/3 as much fuel as the idle jet. But Dellorto accell jets are pumped, so they provide more fuel. Anyway, be that as it may, where in our Mikunis or Hitachis would this extra fuel come from? One thing is pretty certain, not from the idle jet...
     
  10. XJ550H

    XJ550H Well-Known Member Premium Member

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    the faster the motor runs the more fuel is drawn in by the vacuum it creates in the carb throat.
    as the intake air rushes past the orifaces it draws in the fuel. venturi effect is what it is called
    as the slide needle lifts out of the emulsion tube it allows more fuel to be drawn up the main jet and air through the air jet
    the air asperates with the fuel making it a mist instead of a stream of fuel to improve the burning.

    pilot mixture screw meters in the fuel for starting the bike and running the bike as the butter flies open up more fuel is sucked into the motor as more air is allowed through the carb throat.
    when there is enough vacuum the slides lift allowing the main jet fuel to come into play

    if you notice when the bike is not running and you twist the throttle no fuel flows.

    the air jets asperate the fuel so you get a mist and not a stream of fuel.

    put a straw in a bottle with water in it and blow across the straw the liquid will rise in the straw

    the idle jet does not provide much fuel but does supply fuel at a varrying rate as does the main jet. all based on how fast the fuel pump on the front of the carbs spins we call it the motor.
     
  11. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    Because the purpose is to provide a transition burst of fuel while waiting for the main jet to become activated via the lifting of the piston/needle. The change in fuel flow thru the pilot jet is basically instantaneous in response to vacuum pressures; the more the throttle is opened the more the airflow the greater the vac draw. The main jet, by contrast, depends on the lifting of the piston, which is: a) heavy, and b) dampened by a spring. So it requires a moment to respond (Newton's law of inertia and all that......) and the pilot circuit helps smooth over this "hole" in the fuel supply.

    I think that your thesis that the idle jet is sized "for progression under steady state" (and only for this purpose) may be a stumbling block.
     
  12. Minimutly

    Minimutly Well-Known Member

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    Please forgive me, but I never read so much tosh in all my life...
    And that's both replies - just why can you not see the physics in this? Oh I know, you refuse to see that these are constant depression instruments...
    Do I give up or persevere? I dunno.
     
  13. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    I'm not sure what "tosh" means, but it sounds like nothing good.......



    What makes you think that your statement above is (totally) true? You are ASSUMING that these are the only factors that were taken into account by Hitachi (and Mikuni) engineering in the design of these carbs. Certainly that is ONE factor, but how do you know that is the ONLY factor?


    Right back at you, big guy!
     
    Last edited: Dec 21, 2020
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  14. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    This discussion is going nowhere.
    Until and unless a Hitachi or Mikuni factory service manual becomes available (highly unlikely as they were not supposed to be sold on the open market) then this will never be settled.

    I suggest that we move on to more productive endeavours.
     
  15. Minimutly

    Minimutly Well-Known Member

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    So you don't agree, can't see it, so suggest to close the discussion - best I not comment..
     
  16. ManBot13

    ManBot13 Well-Known Member

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    There are some good cutaway pics that show the idle mixture screw and circuits: https://www.xjbikes.com/forums/threads/something-new-something-naked.36344/

    Again, this isn't "confirmed" but it make sense that the idle air and fuel jets are sized for off idle fueling (accell fueling) and the idle mixture screw independently steps that down for the idle port. So while off idle and idle fueling are not "independent" there are enough free variables to get the specific idle performance, and an off idle performance that is "good enough." One thing to remember is that once you blip the throttle, you can't think of anything in terms of steady state, it's going to be a dynamic system. Steady state would only apply once the slide stops moving.
     
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  17. Minimutly

    Minimutly Well-Known Member

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    Yes correct, a dynamic system that is fuel hungry, needing around 2/3rds the amount the idle circuit was giving extra...
    And this comes from a circuit everyone sensible would agree is callibrated to give perfect fuelling for off idle/cruise? No way...
     
  18. ManBot13

    ManBot13 Well-Known Member

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    I sense that minimutly is having some specific problems with his carbs. I think it might be helpful to know what those are. I'm sure everyone on this site has gotten to a point where they think CV carbs just don't work, but there's plenty of others with evidence that they do.

    I think some of the confusion is with how the different parts of the carbs interact and are adjusted for performance:

    Like XJ550 said, the mixture screw affects the idle mixture (EPA and all that); however, the secondary circuit is impacted by the pilot fuel and air jets because it's in the pilot circuit. But minimutly is correct that the screw is down stream of the secondary passages in the carb throat, so would not have a direct impact on the mixture (I hadn't realized this before).

    To Mr Seca's final question, he does not specify when he is "running rich." At idle? Off idle? On the Main circuit? What is his evidence for running rich (spark plug color, soot, popping/back firing, etc)? Why might he be running rich? If he's running rich at idle, the idle screws caps have been drilled out, and the PO never got it to run right, airbox is still there and it runs fine at speed? Sure, give the idle screw a turn. If he put pods on, and now his exhaust is covered in soot and every time he rides he smells like unburnt exhaust? Pilot screw won't do much there.
     
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  19. ManBot13

    ManBot13 Well-Known Member

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    So you mention accel jets in a Dellorto carb, are those slide carbs or CV carbs? I know from my Harley Super E carbs that they have an accelerator pump, and it's basically a cam on the throttle linkage that gives a "squirt" of fuel when you whack the throttle open. This is necessary because opening the venturi before the engine speed changes (whacking the throttle open) actually drops the speed of the air (same flow rate, more area) and lowers the vacuum in the venturi. This is the reason for the large jet and accelerator pump. And you can actually flood the engine while it's off if you twist the throttle a bunch of times. That doesn't happen on an XJ (any idiot can sit on it and go VROOM VROOM when parked and it'll start up just fine).

    Constant Velocity carbs restrict the venturi opening at the slide according to a balance between the vacuum in the venturi and the spring pressure behind slide. This restriction maintains the speed of the fluid in the venturi even when the throttle plate opens, only rising as the engine speed rises and pulls more vacuum. That's why you don't need such a big squirt of fuel when you whack the throttle on a CV carb.
     
  20. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    This just isn't a productive discussion at this point, and it's not directly related to the question that the OP asked.
     

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