Paging a carb whiz (Lozzo?)

Discussion in 'Classic Motorbikes' started by The Older Gentleman, Nov 14, 2003.

  1. The CD200 Benly has its pilot air screw in the base of the float bowl,
    screwing vertically into the bowl.

    I'm just setting up the carb, and due to a K&N air filter and pattern
    cans it's a *tad* lean.

    With this set up, am I right in thinking that screwing the adjuster *in*
    richens the mixture?

    Haynes BoL says two turns out. I'm thinking 1.5. Of course, in this
    position, it's a bugger to get to ut I have a mini ratchet screwdriver
    that will reach it.
     
    The Older Gentleman, Nov 14, 2003
    #1
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  2. The Older Gentleman

    Pip Guest

    On Fri, 14 Nov 2003 23:51:26 +0000,
    Yes, I believe so. As I understand it, the way that air screws work
    is that they allow a flow of air past the tapered needle/seat at the
    business end.

    The further you screw it in, the thicker the needle in the flow/closer
    to the seat you get. This richens the mixture effectively, by
    reducing the amout of air passing the screw/needle/seat.

    That's the way I remember it, anyway. I always have to sit down, have
    a fag and think about it before I do it ... then rehearse in the air
    which way the bloody screw turns before taking a tool to it.

    HTH.
     
    Pip, Nov 15, 2003
    #2
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  3. The Older Gentleman wrote
    Has Sir ever considered investing in a brace of offset screwdrivers?

    Very useful in tight corners they are. And they can make a useful
    temporary substitute for the thing in Swiss army knives designed to get
    boy scouts out of horses hooves.
     
    steve auvache, Nov 15, 2003
    #3
  4. I've got some, but this screw is so placed that it's about half an inch,
    maybe an inch, above the crakcase top. Not enough clearance for my
    offset driver.

    This little ratchet screwdriver/allen key set is made by Cartwright of
    the USA (never heard of them). It is *sm* small and neat. I found it in
    the toolkit of a bike I once bought.
     
    The Older Gentleman, Nov 15, 2003
    #4
  5. Heh. You and me both.
     
    The Older Gentleman, Nov 15, 2003
    #5
  6. The Older Gentleman

    OH- Guest

    4 stroke, isn't it ? Turning the screw out should make the mixture richer.

    At idle, the air flow is split into two streams. One part goes past the
    throttle
    or slide and is supposed to pick up negligible or no fuel at all. In other
    words
    it is extremely lean.
    The other part of the airflow is through the idle system. The amount of air
    passing this way is regulated by the adjuster we're talking about. The air
    that passes the idle system picks up fuel on the way. The mixture from the
    idle system is always rich, it needs to be because it will mix with the
    other
    stream ("throttle bypass") that is very lean. How rich the idle system
    mixture
    gets is determined by the idle jet.

    All you do by adjusting the idle "air" screw is to influence the relative
    amounts
    of rich idle system flow and lean throttle bypass (adjustable by the
    throttle
    stop screw).
    I think this explains why you need to change the idle jet when reasonable
    adjustment of the idle adjustment screw can't provide a good idle tuning.

    The adjustment of the throttle stop screw can effect the idle mixture not
    only
    by allowing more air past the throttle but also by reducing the pressure
    differential that is driving air through the idle system (inlet upstream of
    the
    throttle, outlet downstream of the throttle.
    This loss of driving pressure differential is of course what makes the idle
    system stop working as you open the throttle and get into the range where
    the slide needle controls the fuel/air mixture.

    2 stroke carburettors are different. And IIRC their adjusters work the
    other way i.e. out = lean.

    This post might be total bollox. OTOH, if nobody tears it to shreds it
    might be that I got it right ?
     
    OH-, Nov 15, 2003
    #6
  7. The Older Gentleman

    Preston Kemp Guest

    If the screw's on the engine side of the float bowl, it's a fuel screw,
    so out to richen. Standard setup for 4 strokes.

    If the screw's on the airbox side, it's an air screw, so in to richen.
    Most 2-strokes have these.

    If it's in the middle of the float bowl (I.e. halfway along 1 side),
    bugger knows, but it's pretty irrelevant anyway. Just find the position
    that gives the fastest tick-over. If screwing it in & out makes no
    difference, you'll need the next size larger pilot jet.

    If you're desperate to know if it's a fuel or air screw, screw it in
    until the engine starts to stumble, then give it partial choke. If it
    stops stumbling, you know it's out to richen (fuel screw).
     
    Preston Kemp, Nov 15, 2003
    #7
  8. <VBG>

    OK, so we have (at the last count) three different interpretations.

    Yup, more or less what I expected.
     
    The Older Gentleman, Nov 15, 2003
    #8
  9. This is what I remember from pilot screws in the sides of the carb
    bodies, yes. Never encountered one in a float bowl before. Thanks.
     
    The Older Gentleman, Nov 15, 2003
    #9
  10. The Older Gentleman wrote
    *The* very best source of some cracking odd little tools.
     
    steve auvache, Nov 15, 2003
    #10
  11. The Older Gentleman

    plutonic Guest

    Me too - then somebody taught me 'Lefty Lucy, Righty Tighty' works every
    time ;-).
     
    plutonic, Nov 15, 2003
    #11
  12. The Older Gentleman

    Pip Guest


    Yes, dear. But when you then have to invert your body, crane your
    neck round and under as you attack the fastener by stretching your arm
    around the other side - clockwise and anticlockwise all go a bit
    quantum, like.
     
    Pip, Nov 15, 2003
    #12
  13. The Older Gentleman

    Pip Guest

    On Sat, 15 Nov 2003 18:11:38 +0000 (UTC), "Dr Ivan D. Reid"

    [clock/anticlock confusion]
    Aye. Direct line of sight, with your fissog in "normal" attitude to
    the fastener really helps.
    That is the Best Cheat Of All - and my reason for buying a Snap-On
    ratchet which at the time was the only one I'd seen with the
    changeover lever clearly labelled Do and Undo.

    My current favourites for confusion are car brake caliper bolts, which
    by their very nature screw towards the outside of the car and
    necessitate reaching round the disc/hub/strut assembly to get to them,
    while kneeling with my head in the wheelarch, looking down at the
    bastard bolts. Can't use my pet 3/8 ratchet on them as the torque is
    a bit excessive for little ratchet teeths, so big spanners doubled up
    for extra leverage is the way to go.

    I have been know to set my ratchet to undo, place it in the plane of
    the bolthead and make the appropriate motion, noting which direction
    it turns in ... then trying to feckin' remember when I get the
    spanners round there ;-)
     
    Pip, Nov 15, 2003
    #13
  14. The Older Gentleman

    Lozzo Guest

    Preston Kemp said....
    Wot 'e said. That's exactly what I was taught many moons ago, when
    CD200s were still being built.

    --
    Lozzo
    ZZR1100D, GPZ500S, CB250RS x3
    BOTAFOT#57/70a, BOTAFOF#57, two#49, MIB#22, TCP#7, BONY#9,
    ANORAK#9, DIAABTCOD#14, UKRMT5BB, IBW#013, MIRTTH#15a/16,
    BotToS#8, GP#2, SBS#10, SH#3, DFV#14, KoBV#3.
    Url for ukrm newbies : http://www.ukrm.net/faq/ukrmscbt.html
    http://www.glfuk.com/ for MJK Leathers in the UK.
     
    Lozzo, Nov 15, 2003
    #14
  15. The Older Gentleman

    sweller Guest

    So I'm not the only one..
     
    sweller, Nov 15, 2003
    #15
  16. The Older Gentleman

    Dave English Guest

    In message
    <1g4geiw.1wii1gg11rbt34N%>, The
    They're the United States of America :-^)
    ....
     
    Dave English, Nov 18, 2003
    #16
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