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

  1. #1
    Senior Member -alan-'s Avatar
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    Mixture screw setting problems- and a flat spot in the carburettion ?

    Gentlemen,

    Following on from this original thread:

    https://www.hondatwins.net/forums/56...setting-2.html

    and a more recent one looking for help on pilot jet assembly, I suspect there's something still not right with the whole pilot jet/idle circuitry. I was wondering whether you might give me pointers as to where to go next ?

    General:

    The bike has a standard K5 engine, with the original air-box and exhaust. I rebuilt the engine to sort low compression issues a couple of years ago. After numerous teething problems (down to my own inexperience and stupidity), it has been fairly usable after the initial period and has put up perhaps about 1000 miles with no major problems since. The carbs were stripped and cleaned a couple of times (by me, with a little home ultrasound cleaner) at this stage as part of the initial troubleshooting. The tappets, timing, cam-chain tension have been checked recently and are within spec.

    The problem/symptoms:

    - The bike starts and runs reasonably well. It will pull strongly above 4k and rev out cleanly.
    - There is, however, a noticeable flat spot when you open the throttle - either quickly or slowly - from 2.5k.
    - Once you are through this, the bike pulls and revs cleanly.

    Other symptoms:

    - Opening one throttle at a time confirms the right hand cylinder is slower to rev, and prone to stuttering, the left is fine.
    - When trying to adjust the mixture screw setting as per the recommended procedure, I notice that whilst the left works as you would expect - the right hand one has negligible effect until it is almost fully in, and then the revs rise progressively over the last 1/2 turn until it is fully home.

    Where Im at at the moment - rightly or wrongly - is assuming that the low-speed flat spot and the mixture screw setting difficulties are connected. I suspect that there's an outside chance the type of mixture screws I have might also be the wrong profile for the right hand carb, based on the fact they dont seem to do anything until they almost seal the circuit in its entirety.

    At a bit of a loss as to where to go next. Any pointers greatly appreciated.

    Thanks, Alan
    Last edited by -alan-; 09-11-2019 at 10:40 AM.
    CB450 K5

  2. #2
    Senior Member WintrSol's Avatar
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    How do the spark plugs compare, soot-wise? The flat spot and pilot screw symptoms say rich on that side, but it could also be that the idle is set higher (out of sync), and the needle jet is contributing more fuel, so the pilot setting is set to less.

    The flat spot may be to a too-full bowl, or another path making the mixture a bit rich, like a loose jet. Basically, when you open the throttle, a CV carb is temporarily richer, as the air velocity over the needle/main jet is higher until after the piston lifts, causing more fuel to pull through the jets until the velocity drops to 'normal'. If the carbs are out of sync, then the left one will pass through this rich zone more than the right, and make for an imbalance. So, verify your idle and throttle sync are very close, before chasing mixture issues.

    You may also have a weak spark plug or spark on that side, and the inrush of air is blowing the spark out. Once the sync and mixture is checked, this would be next on my list.
    Rick

    Mine is a mostly 1970 CB450K3

  3. #3
    Senior Member -alan-'s Avatar
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    Quote Originally Posted by WintrSol View Post
    How do the spark plugs compare, soot-wise? The flat spot and pilot screw symptoms say rich on that side, but it could also be that the idle is set higher (out of sync), and the needle jet is contributing more fuel, so the pilot setting is set to less.

    The flat spot may be to a too-full bowl, or another path making the mixture a bit rich, like a loose jet. Basically, when you open the throttle, a CV carb is temporarily richer, as the air velocity over the needle/main jet is higher until after the piston lifts, causing more fuel to pull through the jets until the velocity drops to 'normal'. If the carbs are out of sync, then the left one will pass through this rich zone more than the right, and make for an imbalance. So, verify your idle and throttle sync are very close, before chasing mixture issues.

    You may also have a weak spark plug or spark on that side, and the inrush of air is blowing the spark out. Once the sync and mixture is checked, this would be next on my list.
    Thanks WintrSol. I always find your explanations educational and enlightening.

    Had only time for a short run, so a lot of the plug colour is indicative of that - but there definitely seems to be an imbalance, with things running richer on the right:



    That ties up with the smell, smelt like it has been running with the choke permanently on.

    From here - logical next step would be to remove the carb, check the float height, pull the jets (and squirt some more cleaner through them as I have the carb off anyway) - and then check, reinstall and tighten all of the jets ?



    edit: One other thought strikes me: if the piston were slower to rise on this RH carb than the other, that too might give rise to that transient over-fuelling situation you describe. I should perhaps give the carb a through cleaning throughout, and do the 'straw test' procedure to cover that off as well.
    Last edited by -alan-; 09-11-2019 at 01:41 PM.
    CB450 K5

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  5. #4
    Senior Member WintrSol's Avatar
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    I would not remove the carb to check the float level, but instead, raise the float with a float gauge and open the petcock. If fuel flows, the level is clearly wrong; if it just moistens the float valve, it's good.
    Yes, a sticking piston could enrich the mixture, especially during transient operation. The straw test allows you to roughly compare both sides, to see that similar pressures raise the pistons similar amounts, and at about the same rate. It could just be the piston cover gasket rubbing on it, delaying the rise just enough.
    Rick

    Mine is a mostly 1970 CB450K3

  6. #5
    Senior Member 76TWIN's Avatar
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    I'll second the straw test as well. The barrel in the carb where the piston travels has to be spotless and smooth. It doesn't take much for it to bind up the piston.
    '76 CB500T Frankenbike
    '73 CL450 (basket case for now)


    Carpe narem.

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    Senior Member -alan-'s Avatar
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    Thanks guys.

    Quote Originally Posted by WintrSol View Post
    I would not remove the carb to check the float level, but instead, raise the float with a float gauge and open the petcock. If fuel flows, the level is clearly wrong; if it just moistens the float valve, it's good.
    Yes, a sticking piston could enrich the mixture, especially during transient operation. The straw test allows you to roughly compare both sides, to see that similar pressures raise the pistons similar amounts, and at about the same rate. It could just be the piston cover gasket rubbing on it, delaying the rise just enough.
    Ill check the float levels that way - a lot easier

    I have a sneaking suspicion the only time I attempted the straw test, both pistons rose, but not at the same rate. If the float levels check out ok, Ill pull the carbs and see what I can find.
    CB450 K5

  8. #7
    Senior Member -alan-'s Avatar
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    So, with a little time free this afternoon, I removed the float bowls to see what the respective fuel levels were like in the right and left carbs:



    The one at the top is off the right hand cylinder, which is the one with the stutter c.2.5k, and the plug showing signs of running rich.

    Not a great photo, and not a very accurate test either, but the fuel level in the RHS bowl is not higher than the other - and if anything is even a little lower.

    From here, I think it's reasonable to assume that the signs of rich running is not down to the float level on the right.

    Based on this, it seems to me the more likely suspects at this stage are bad spark/poor burning on the right - or the piston being slow to raise.

    Qn: Is this a fair assumption - and if so, what's the best next step ?
    CB450 K5

  9. #8
    Junior Member KYCB360T's Avatar
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    The mixture screws are only in effect in the idle circuit. As soon as you crack the throttle, they are not in play anymore. At 2500 rpm the main jets take over and are the main contributor to the fuel mixture. At around 4000 rpm, the slide will come in play and will lift to open the main jet.

    Have you checked or synchronized the carbs? This should be verified first. There are two things to check during synchronization; 1) make sure the idle screws are producing the same level of vacuum on your gages and 2) make sure that the vacuum rises the same on each side when you crack the throttle. Any imbalance in the above will greatly impact the performance.

    It looks like you have one good running cylinder and the other one is playing catch-up, but it is not winning!

    Attached is a diagram to help understand the workings of these carbs.Mixture screw setting problems- and a flat spot in the carburettion ?-450-carb.pdf
    Last edited by KYCB360T; 09-13-2019 at 10:44 AM.

  10. #9
    Senior Member -alan-'s Avatar
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    Yeah it definitely feels like it's playing catch up as the revs rise.

    I've studied that carb diagram ad-nauseum. Interesting - but probably raises more questions than answers at the moment.

    And there's no vacuum gauge facility with the 450 alas.. That would much too easy ..
    CB450 K5

  11. #10
    Junior Member KYCB360T's Avatar
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    Yeah, the 500's have them built into the carbs. I've seen guys drill and tap into the carb manifold. Easier said than done.

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