CV carb question

Discussion in 'The Garage' started by uk_mouse, Mar 25, 2009.

  1. uk_mouse

    uk_mouse Aquatic adventurer

    Joined:
    Jul 20, 2006
    Oddometer:
    1,632
    Location:
    UK
    Can someone answer this for me please? I'm wondering how a certain parameter affects the behaviour of CV carbs ... the carbs I'm looking at are the Keihins used on the KTM 950 but the question is applicable to any constant depression carb, I think.

    On the bottom of the slide, next to where the needle is mounted is a hole (or two but functionally it's the same thing). The vacuum created in the venturi sucks air though this hole, causing a vacuum above the diaphragm, thus causing the slide to lift up, thus lifting the needle and allowing more fuel to flow out of the needle jet.

    So far so good. But how does the size of this hole affect the behaviour of the slide? If the hole is smaller, will the slide lift more slowly, or will it just lift less? Is changing the size of the hole equivalent to moving the needle, or is it more complicated than that? (I suspect the answer to that last one is yes!)

    Thanks for any enlightenment you can send my way :)
    #1
  2. fritzcoinc

    fritzcoinc Enjoying my last V8

    Joined:
    Apr 30, 2008
    Oddometer:
    8,011
    Location:
    Hockley, Tx
    A larger hole provides a quicker movement of the slide but to the same opening point. Quicker opening may require other adjustments to over come a lean condition that may be a result of the quick opening.
    #2
  3. uk_mouse

    uk_mouse Aquatic adventurer

    Joined:
    Jul 20, 2006
    Oddometer:
    1,632
    Location:
    UK
    Thank you ... as I understand it, slowing the movement of the slide will actually create a temporary enrichment when the throttle is opened? This is counter intuitive as you'd expect the mixture to richen as the needle rises, but the slide opening actually reduces the venturi effect, thus sucking less fuel out of the jet. Am I correct there?

    What I'm getting at is .. by blocking one of the vacuum holes in the slide, I am not going to mess up the mixture on a steady throttle, but it will create a slight "accelerator pump" effect?

    Thanks again for the info!
    #3
  4. fritzcoinc

    fritzcoinc Enjoying my last V8

    Joined:
    Apr 30, 2008
    Oddometer:
    8,011
    Location:
    Hockley, Tx
    Don,t think so.
    What your describing would be like running an engine with a cloged air filter.
    If you slow down the vac slide, throttle response will be dismal.
    #4
  5. bomber60015

    bomber60015 Anatomically Correct

    Joined:
    Sep 11, 2008
    Oddometer:
    26,616
    Location:
    Chicago-ish
    +1 -- if you need to richen the mixture at a specific throttle setting, I suggest you address that in a more traditional manner (shim the needle or move the clip, change needle profile, et al) . . . .
    #5
  6. motu

    motu Loose Pre Unit

    Joined:
    Dec 28, 2001
    Oddometer:
    4,902
    Location:
    New Zealand
    On the SU and CD Stromberg the speed of the piston lift could be adjusted by different weight oils in the dashpot.This acted as the accelerator pump - slowing the piston down increases air flow over the jet,sucking out more fuel and increasing the mixture.Lighter oil raises the piston faster,slowing air speed over the jet and giving less fuel.This is only for acceleration,as once the piston gets to where it should be,mixture strength will be as it should be.It's a pity motorcycle carbs don't have a proper dashpot.
    #6
  7. uk_mouse

    uk_mouse Aquatic adventurer

    Joined:
    Jul 20, 2006
    Oddometer:
    1,632
    Location:
    UK
    Yes, I was thinking of the way the SU carbs work when I was considering this question. I may try it anyway, I can always drill the holes out again if I don't like the results :) What got me started on this line of thought was that the slides on my carbs have had the holes drilled out to a larger size sometime in the past, and I was wondering if it made the response a bit too twitchy...
    #7
  8. Skippii

    Skippii Milkshakes, my lad.

    Joined:
    May 11, 2008
    Oddometer:
    6,571
    Location:
    Richmond, Va
    The hole is the compression damping of the slide. It doesn't change the travel, only the sensitivity.

    Just as with your shock, too little damping (big orifice) means the slide overshoots its target and can even oscillate up and down before settling in the correct spot.
    #8
  9. concours

    concours WFO for 44 years

    Joined:
    Dec 2, 2008
    Oddometer:
    5,684
    Location:
    Kensington, NH USA
    Slowing the speed at which the slide opened when requested was the entire reason we transitioned to CV in the first place. When an instantaneous "WHACK" to WOT occurs at the twist grip, The CV carb opens AS FAST AS THE ENGINES AIRFLOW CAN TOLERATE (properly calibrated by the OEM, of course) Conventional carbs wil open too fast, vacuum in the venturi goes to (near) zero, fuel is no longer sucked from the delivery tube and a lean BWOOOOOG occurs. Accelerator pumps were added on some (mimmicking automotive design) but the CV works way better. If you are looking to modify this orifice size, think again.
    #9
  10. XDragRacer

    XDragRacer Long timer

    Joined:
    Mar 25, 2009
    Oddometer:
    3,811
    My understanding, the hole(s) in a CV carburetor slide is (are) a "vacuum" (pressure differential) port. Enlarging the port may transfer this pressure differential ("depression" from atmospheric pressure) from the venturi to the mixing chamber above the diaphragm more quickly than the OEM-sized orifice.

    While I didn't instrument and measure the effect, drilling out my CVK40 Keihin slide to 7/64" diameter enhanced throttle response, measured on calibrated seat-of-the-pants meter.

    Ersatz "accelerator pump" effect by blocking partially the hole? I don't think so; an accelerator pump dumps raw fuel into the venturi, enriching the mixture upon slide opening to enhance throttle response; by contrast, obstructing the vacuum port(s) would only slow the slide response to venturi pressure differential, as far as I can tell; thus delaying raising the needle and consequently delaying the uncovering of the needle and the main jets, delaying fuel supply delivery to the mixture.

    In other words, partially obstructing the vacuum port(s) would inhibit throttle response, the opposite effect of an accelerator pump, boring-out the vacuum port(s), or even clipping coils off the slide spring, IMHO (YMMV).

    Apologies if the following link's redundant or cliche'; I found it a useful CV carburetor tutorial:

    http://www.gadgetjq.com/keihin_carb.htm
    #10
  11. uk_mouse

    uk_mouse Aquatic adventurer

    Joined:
    Jul 20, 2006
    Oddometer:
    1,632
    Location:
    UK
    But why were accelerator pumps introduced in the first place? As I understand it, when the throttle opens and the slide rises, there's a temporary drop in fuel delivery, because the larger venturi (created by the position of the slide) causes a drop in vacuum and thus less fuel is sucked through the jets. The acc. pump is a bodge designed to mask this temporary lean condition.

    An SU carb (as described above) has an oil-damped slide, thus eliminating this lean spot as the slide moves more slowly.

    The "ersatz accelerator pump" question was not really my main train of thought, I mainly wanted to clarify the function of the vacuum port in the slide, so thanks to everyone who's posted info :)

    As I said earlier, the slides in my carbs have been drilled out in the past. I find the bike's response to be a bit too "twitchy", which makes it tiring to ride in traffic for example. So I was going to return the holes to the standard diameter if I can, then I thought while I'm at it, why not experiment, just for the sake of it really? :)

    Thanks for the link, it's always good to read more about the subject.
    #11
  12. fritzcoinc

    fritzcoinc Enjoying my last V8

    Joined:
    Apr 30, 2008
    Oddometer:
    8,011
    Location:
    Hockley, Tx
    When the throttle is opened in any type of crab. it takes a little time for the fuel to get moving through the various passages creating a breif, but very objectionable, lean condition.

    An accelator pump is used in NON CV carbs. to squirt raw fuel into the engine to over come the lean condition created by the opening throttle. The CV type with a vac. operated slide is another way, some think better, to eliminate the same lean comdition at throttle opening. An accelator pump type carb. mnay produce more emmisions because the pump shot is fixed and may be too much fuel for some throttle opening conditions and maybe not enough for others. The vac. operated slide in the CV type can adjust to a much broader range of throttle opening conditions since its operation is controlled by the engines intake air demands.

    In general most folk incresae the size of the hole in the CV slide. This will allow the slide to respond more quickly to changes in throttle position. But if the hole is too big the CV slide will open too quickly and lean condition will occur just as it does in a non CV type carb. when the throttle is opened. Larger jetting can possibly be used to over come this.
    The best apporach to use, IMHO, when modifing a CV slide is to drill additional holes in the CV slide that increase the original holes area by 10% at a time. The reason being that it is easer to plug a smaller hole if too many are present. Some CV carbs. have replaceable vent jets to do this.

    Perhaps you should tell us what bike your working on, what are the driveability problems you are experiencing, and what end you wish to acchive.

    Cheers Mate!
    #12
  13. vfxdog

    vfxdog Been here awhile

    Joined:
    Jan 5, 2007
    Oddometer:
    641
    Location:
    Santa Monica or London, depending on work.
    Some CV carbs DO have accelerator pumps- the units fitted to the Honda CBX, for example.

    Slide drilling is a common requirement for aftermarket rejetting kits- Dynojet for instance used to actually include a drill bit in their jet kits. As has already been pointed out, the main criteria for the hole size is one of response- a smaller hole works like a damper for the slide. If you do make the hole too big it's easy enough to sleeve back down- just make sure there's no way that sleeve can come out.
    #13