Mixture screw setting problems- and a flat spot in the carburettion ? - Page 3
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Thread: Mixture screw setting problems- and a flat spot in the carburettion ?

  1. #21
    Senior Member WintrSol's Avatar
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    This schematic shows the carb in the idle position. The slow air jet (SAJ) introduces air above the slow jet (SJ). The pilot jet (PJ) draws fuel from the main jet (MJ), and meters it into the slow jet, which has air holes to mix air from the slow air jet. The bubble effect breaks up the mixture so it sprays better when it exits through the transition ports (13) and past the pilot screw (PS) through the pilot port (12). Without the bubbles (main has them too), the fuel squirts as a liquid out the transition and pilot ports, so doesn't turn into the fine spray that works best. Note that the throttle plate is actually fully closed; the throttle plate doesn't seal at idle, as shown, and passes a bit of fuel/air from the transition ports, which the pilot screw trims. BTW, when you do the straw test, you push air into the air input port for the piston (a), which creates the same effect as vacuum from air velocity passing under port (b), and also puts some pressure on the slow air jet.
    Mixture screw setting problems- and a flat spot in the carburettion ?-carb_schem.jpg
    in operation:
    Mixture screw setting problems- and a flat spot in the carburettion ?-carb_operation.jpg
    Last edited by WintrSol; 09-15-2019 at 02:08 PM.
    Rick

    Mine is a mostly 1970 CB450K3

  2. #22
    Senior Member -alan-'s Avatar
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    Rick- many thanks for that, appreciated.

    Quote Originally Posted by WintrSol View Post
    This schematic shows the carb in the idle position. The slow air jet (SAJ) introduces air above the slow jet (SJ). The pilot jet (PJ) draws fuel from the main jet (MJ), and meters it into the slow jet, which has air holes to mix air from the slow air jet. The bubble effect breaks up the mixture so it sprays better when it exits through the transition ports (13) and past the pilot screw (PS) through the pilot port (12). Without the bubbles (main has them too), the fuel squirts as a liquid out the transition and pilot ports, so doesn't turn into the fine spray that works best. Note that the throttle plate is actually fully closed; the throttle plate doesn't seal at idle, as shown, and passes a bit of fuel/air from the transition ports, which the pilot screw trims. BTW, when you do the straw test, you push air into the air input port for the piston (a), which creates the same effect as vacuum from air velocity passing under port (b), and also puts some pressure on the slow air jet.
    Really sorry about so many questions - but this is the bit that has me puzzled: when trying to set the carb up at idle: is it supposed to be operating:

    as per diagram (a)
    - i.e with the throttle plate fully closed and air seemingly flowing through the transfer ports into the slow jet circuit circuit and then trimmed by the pilot screw

    or ..

    as per diagram (b)
    - with the throttle plate cracked open a small fraction, and fuel from the slow-jet circuit flowing through the transfer ports in the opposite direction out into the main Venturi ?
    CB450 K5

  3. #23
    Sensei 66Sprint's Avatar
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    Diagram B is BELOW diagram A, NOT to its right........

    There is NO reverse direction flow EVER unless caused by sudden closure turbulence, and even then unlikely it would get back to the venturi area under the slide piston......
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."
    Contact info: E-mail; [email protected] Phone; 540-525-5199

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  5. #24
    Senior Member -alan-'s Avatar
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    Quote Originally Posted by 66Sprint View Post
    Diagram B is BELOW diagram A, NOT to its right........

    There is NO reverse direction flow EVER unless caused by sudden closure turbulence, and even then unlikely it would get back to the venturi area under the slide piston......
    I'm aware diagram (b) is below (a). The reason I asked about whether air flowed from the carb throat into the slow jet circuit was because of the downward pointing arrow at the base of the throttle plate and to its left in (a).

    I hear what you are saying about the flow-direction, thanks 66sprint. My apologies if I've worded this wrong.

    My question is basically down to whether the mixture screw adjustment can only be done with the throttle plate closed (or as near as possible) with zero fuel flow through the transfer ports) as in the running condition shown in (a) or..

    Will the mixture screw still work properly and be able to satisfactorily regulate the mixture / raise lower the rpm if the throttle plate is not fully closed as in running condition shown in (b) ?
    (ie if my throttle plate isn't closing and sealing properly, should I still be able to fine-tune the idle mixture with the pilot screw)

    Hope that hasn't added extra confusion
    Last edited by -alan-; 09-15-2019 at 03:57 PM.
    CB450 K5

  6. #25
    Senior Member WintrSol's Avatar
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    Diagram (a) shows that some air passes through the throat of the carb and past the throttle plate, but is moving too slowly to pull fuel from the main/needle jet, so nearly all the fuel mixture comes via the pilot circuit, and most is controlled by the pilot screw. The arrows show that some air is getting around the top and bottom of the throttle plate. Diagram (b) shows what happens as the throttle begins to open; there is still little to no fuel coming from the main/needle jet, but more pilot mixture through the transition ports, to keep up with the extra air. There is only one arrow to the right for the combined air flow right of the throttle, as pointer (13) conflicts with the arrow in (a). Diagrams (c) and (d) show no fuel coming from the pilot circuit, but there is always some, just not enough to be significant.
    Rick

    Mine is a mostly 1970 CB450K3

  7. #26
    Sensei 66Sprint's Avatar
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    Quote Originally Posted by -alan- View Post
    I'm aware diagram (b) is below (a). The reason I asked about whether air flowed from the carb throat into the slow jet circuit was because of the downward pointing arrow at the base of the throttle plate and to its left. YES, but that is shown ONLY AT IDLE in diagram A.... I hear what you are saying about the flow-direction, thanks 66sprint.

    My apologies if I've worded this wrong.

    My question is basically down to whether the mixture screw adjustment can only be done with the throttle plate closed (or as near as possible) with zero fuel flow through the transfer ports) as in the running condition shown in (a) or..

    Will the mixture screw still work properly and be able to satisfactorily regulate the mixture / raise lower the rpm if the throttle plate is not fully closed as in running condition shown in (b) ?
    (ie if my throttle plate isn't closing and sealing properly, should I still be able to fine-tune the idle mixture with the pilot screw)

    Hope that hasn't added extra confusion
    No, NOW it makes more sense.....The mixture screw CAN be adjusted at any time, but unless the throttle plate is closed or almost closed, it WILL be hard to determine what if any difference it is making as its effect diminishes inversely as the throttle opens....Once the throttle plate opens to the point where the drawn air volume can all go straight under or over the plate, the idle circuit (and mix screw setting) is essentially removed from the equation and adjusting it would have ZERO effect.......
    Last edited by 66Sprint; 09-15-2019 at 10:57 PM.
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."
    Contact info: E-mail; [email protected] Phone; 540-525-5199

  8. #27
    Senior Member WintrSol's Avatar
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    Which brings us back to carb sync, but at idle. It doesn't have to be perfect, but close.
    Rick

    Mine is a mostly 1970 CB450K3

  9. #28
    Senior Member -alan-'s Avatar
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    Quote Originally Posted by 66Sprint View Post
    No, NOW it makes more sense.....The mixture screw CAN be adjusted at any time, but unless the throttle plate is closed or almost closed, it WILL be hard to determine what if any difference it is making as its effect diminishes inversely as the throttle opens....Once the throttle plate opens to the point where the drawn air volume can all go straight under or over the plate, the idle circuit (and mix screw setting) is essentially removed from the equation and adjusting it would have ZERO effect.......
    I think the penny has finally dropped ..

    If the slow idle circuits are doing their job properly, then the bike should be able to idle with the throttle plate fully closed.

    If that's not happening, then there's a problem either with the amount of fuel being supplied through the idle jet, or the volume of air coming through the slow air jet circuit.

    That gives me something to work with

    Thank you one and all for your perseverance and help gentlemen. I will report back..
    CB450 K5

  10. #29
    Senior Member tbpmusic's Avatar
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    Quote Originally Posted by -alan- View Post
    .....

    If the slow idle circuits are doing their job properly, then the bike should be able to idle with the throttle plate fully closed.

    .
    I'm not sure that's entirely true - that's what the idle screws are for, to open it just a bit.
    WintrSol likes this.
    Bill Lane
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    '75 CB200T, '71 CB450 K4 Half-Breed, '72 CL350 (Sold), '81 CM200T, '72 C70M
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  11. #30
    Senior Member WintrSol's Avatar
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    Quote Originally Posted by -alan- View Post
    If the slow idle circuits are doing their job properly, then the bike should be able to idle with the throttle plate nearly closed.
    Fixed that - if the throttles are fully closed, the engine rpm will drop to a stall.
    Rick

    Mine is a mostly 1970 CB450K3

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