"Beefier" Airhead Diaphragm

Discussion in 'Airheads' started by snoop, Mar 5, 2008.

  1. squish

    squish Out of the office.

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    Right then
    So the throttle (butterfly) controls the air into the engine.
    As the throttle is opened, and engine speed increases, there is a pressure differential between the throat of the carb (the bottom of the slide) and the area above the slide's diaphram.
    This vacuum, (that's the V in CV carbs to some people, or the Depression in CD carbs) lifts the slide, this slide lifting, pulls the needle up, the tapered needle allows more fuel to flow up the emulsion tube and into the intake stream.

    The amount of fuel flowing into the stream is controlled by this slide and needle combination. (This is assuming that we have moved off of fuel being metered by the idle circuit.)

    The amount of air is controlled by the throttle (that's the butterfly)
    As the throttle is opened more, the engine speed increases, as the speed increases more air is flowing through the throat of the carb, as more air moves, the slide moves up and as a result more fuel is flowing into the intake stream.
    Close the throttle and air is reduced, the slide drops the needle moves deeper into the emulsion tube the amount of fuel is reduced.
    A heavier slide will fall faster, shutting off the fuel flow quicker.

    Frankly this discussion is about CV carb'd bikes not FI but since you bring it up. I don't know about the VFR1200 but I do know that the VFR800FI
    Does not have a CV device, It has four "traditional" (has FI been around that long, we can now call it traditional?) FI throttle bodies with butterfly valves and injectors.

    There is a desire by manufactures to make bikes easy to ride.
    By smoothing out the powerband and making the relationship between throttle inputs and reactions of the bike easy to get the hang of.

    Some bikes (most notable is Suzuki's line of GSXR and DL's) employ a second throttle body, this is to change the intake velocity allowing for a better more complete intake charge at the smallest of throttle openings.
    This is also done to keep intake noise down.

    Many riders have reported disconnecting this device with no ill effects.

    But those second throttle butterflies are not a CV or CD type device.

    The CV or CD describes a Carburetor that has a slide that lifts by way of Vacuum (or Depression if you will),
    This separates from a Slide Carburetor (like what's on the R90S stock) which have slides that are lifted directly by the throttle, or Fixed Venturi Carburetor (older Harleys and many lawnmowers) that don't have a venturi that changes sizes.

    Not surprising since BMW made a number of different bikes for a number of different uses.

    I've never said that heavier slides don't effect the way an engine responds.
    Only that heavier slides don't make a bike more rich.

    For instance if you want more lower end tractable power, it's easy to see that slightly slower reacting carbs would be a bonus, smoothing out abrupt throttle inputs and the like. The drawback is on closed throttle there tends to be an increase in engine braking the faster fuel is shut to the cylinders.

    Where as on a sporty bike ridden on road you might want faster reaction to throttle inputs, give the bike gas it goes roll off it slows...
    Again, the lighter slides allow for a slower closing of the throttle
    and a little less compression braking.

    Also different countries have different tastes when it comes to engine response.
    #21
  2. squish

    squish Out of the office.

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    Right, the butterfly is only the valve, the engine is what pumps the air.
    #22
  3. Rucksta

    Rucksta SS Blowhard

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    Yes

    The engine controls how much air is pumped , the butterfly controls how much air can be pumped (up to the demand limits of the engine) the slide contols the velocity of the air through the carby and the venturi effect (modified by jets, needles, taper, emulsifiers etc) controls how much fuel can enter to engine.

    Oh the fuel (when mixed with the right amount of air) controls the size and speed of the bang which is what cuases the engine to demand more air. Think diesel.

    In my mind when I break it down to basics Beemerboff's "upside down" ideas of carb tuning start to gain some credibility.

    I could have used backward or arse about to describe Beemerboff's ideas but that may have conveyed an opinion I wasn't expressing.:D
    #23
  4. Wirespokes

    Wirespokes Beemerholics Anonymous

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    Isn't it funny how logic can take us down opposite paths? I'd have thought the same thing - that stiffer diaphrams and springs would make for a leaner mixture, contrary to Beemerboff's experience.

    Beemerboff - I see the logic in your explanation and it makes sense. Thanks for being willing to be scoffed at in order to express your contrary viewpoint!
    #24
  5. squish

    squish Out of the office.

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    So I'm totally perplexed on this.

    Just to make sure we are on the same page.
    The way I was taught, transition refers to roughly 1/4 throttle applied to a quarter turn throttle, or roughly the point at which the the slide begins to take over fuel metering from the idle jet.

    At this point, there is still fuel flowing from the idle circuit and fuel is flowing
    from the main as well.

    So your premise is the slower rising throttle, reduces the diameter of the venturi which in turn then raises the air speed, which in turn pulls more fuel up from the float.
    Regardless of throttle butterfly opening.

    Or are you saying that the reduced venturi size effectively pulls more fuel from the idle circuit?

    Is this what you are saying in a nut-shell?
    #25
  6. rufusswan

    rufusswan Been here awhile

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    Human logic will do that. Luckily, CV/CD carbies use the logic provided by the laws of physics.

    The transition to & from idle circuit to main circuit happens each time, every time, at the same time, for exactly the same reason. It is irrelevant how 'beefy' the carb diaphram is.

    Beefier diaphrams, thicker springs, or heavier dashpot oit (for you SU guys) will slow down the time it takes for the needle to react to your throttle inputs. The only thing that changes is time.
    #26
  7. BMWzenrider

    BMWzenrider The Road Scholar

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    I think that I see where the confusion is coming from...
    YES, the taper on the needle does allow for more fuel to get sucked out at the same venturi pressure when the slide rises. But that is not what we are talking about.

    Forget about the needle for a minute.

    Think for a moment about a cheap lawnmower carburator, or even an expensive Holley 4-barrel downdraft....
    Both of those are "FIXED" venturi carburators with no metering needles.
    Yet both designs quite effectively provide varying amounts of fuel from the fixed jets in the venturi throat.
    The variable is the SPEED of the air moving through the venturi that alters the vacuum pressure in the fuel jet.
    The change in velocity through the venturi in these carbs changes how much fuel flows from the main jets.

    That is EXACTLY what is happening for the short time while the heavier slide has not moved to its final balanced position yet...

    For a little while you are getting increased velocity through the venturi area which reduces pressure in the airstream versus what is normally present. That increases the vacuum on the main jet and draws more fuel, even though the flow area is not changing (needle has not moved significantly).
    Just like in that lawnmower or Holley setup...

    Does that explain it a bit more clearly?


    Once the slide moves up to its normal position balanced by the pressure/vacuum differential, the metering flowrate is back to what was calculated for the given needle/jet, so fuel flow goes back to 'normal'.


    I don't disagree with you.
    However, the discussion has been centered around what is happening in the throat of the venturi (at the slide).
    Trying to think about the butterfly position in all of this will just confuse you. It is not the issue...


    Exactly! :deal
    Because the stock needle was selected to prevent the mixture from going too lean while accelerating, where you need a rich mix. But you do not need to be that rich when just cruising.
    So being able to tweak the slides to slow the rise is like adding an accelerator pump. The temporary increase in the venturi effect allows for richer mix when revving up the motor, but still allows for a leaner cruising mix. :clap


    Not sure why you had trouble finding a reference to this trick.
    Found this on the second link that I clicked on when I did an online search for SU spring rate...
    http://www.jetlink.net/~okayfine/su/sumain.html
    "Dome Spring Rate Calculation
    Another method of fine-tuning your SU carbs, especially with a modified motor, is changing the dome spring for a stiffer or softer spring. A stiffer spring will resist the upward movement of the piston, as does higher weight oil/fluid in the dampeners. This resistance to upward movement of the piston provides a degree of fuel enrichment for acceleration.
    "

    I didn't bother to search for other references, but I am confident that they exist...
    Haynes actually publishes a separate manual dedicated to the very similar SU carbs. It goes into operating theory, and I believe has a section on needle/spring selection.
    #27
  8. Beemerboff

    Beemerboff Long timer

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    What we seem to be forgetting here is that fuel does not just flow up through the needle jet on its own accord - the float level is set well below the jet outlet.

    It is sucked up , and the rate it is sucked up depends on the speed of the air flow between the botom of the slide and the top of the needle jet, or the atomimiser or whatever on top of the jet.

    And that speed depends on the relationship between the butterfly position and the slide position - large butterfly opening and small slide opening equals high speed , more suction and a rich mixture, tempered somewhat by the fact that the lower needle position will slow the fuel flow down a bit.

    Bings are a crude carb, and most Airheads run far to rich at most speeds - cruising the interstate on a dead throttle at the state speed limit my injected car gets better consumption than the big GS.

    What I was suggesting was a way to help to clean up the carburation , providing a crisper throttle response and a bit better consumption at the same time.

    I have tried it and it works for me, but there are plenty of other areas where I get different results from the mainstream, and I find that rather fun.

    And fun is what biking is all about for me.
    #28
  9. outfit

    outfit Been here awhile

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    Throughout the years, BMW has apparently used two different types of springs in the Bing carburetors.
    [​IMG] The BMW spare parts catalogue tells that until end 1984, a spring with 115 mm length and about 30 turns has been throughout their model range, from the R45 through R65 up to the R80 and R100. The part number was 13 11 1 335 324. From 1985 on, the spring changed to 20 turns and an overall length of 120 mm (part number 13 11 1 338 134). Again, the same springs are used in all models, both the 40-mm Bings (Type 94) of the R100GS and R100R, and the 32-mm (Type 64) of the "late" R65, R80, and R100.
    An interesting experiment is to put the "new" springs into the "old" carburetors.
    Following a detailed report from Rainer Restat who tried this on his R80G/S, I took the carburetor springs from my trusty R80GS and put them in the R100TIC. The result was ... a smoother idle and a better response to the throttle. In particular in the range between 2000 and 3000/min there was a noticeable difference, which is exactly the zone that I frequently use when strolling through the countryside. At higher rpm, there is no difference.
    The "new" spring is harder than the "old" one, so a probable explanation is that the new spring reduces the pressure fluctuations that occur at low rpms inside the carburetor. A possible reason for the better "response" at low throttle is that the harder spring yields a reduced efficient cross-section when you open the throttle. This yields a higher air intake speed, which in turn leads to better filling and thus more "punch".
    In conclusion, changing the "old" springs against the new ones is a very simple "tuning" which brings better idle and smoother acceleration at the low end. In addition, the new springs are far from expensive (about 10 CHF or 6.50 € a pair), so I really recommend this exchange.
    Many thanks to Rainer Restat for this hint and his detailed report!
    #29
  10. Beemerboff

    Beemerboff Long timer

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    Amen
    #30
  11. squish

    squish Out of the office.

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    To which Beemerboff wrote

    AH HA!. Now I understand where you are getting your logic.

    You are confusing a more complete intake charge with a more rich intake charge.

    What's going on with the smaller diameter venturi is a higher intake charge velocity, which in turn crams both more air and fuel into the combustion chamber.

    I don't disagree a whit that changing the "weight" of a slide changes the effects and the response of a carb.

    What I disagree with is that a heavier "weight" makes the charge more laden with fuel. There is no physical way it can. The needle at the bottom of the slide blocks more fuel from coming up the tube.

    Short of putting a carb into a test fixture to prove this and posting the results I'm done with this discussion.

    Thanks for your time and the explanation of your theory.




    as for the assertion that the throttle doesn't matter. Well it does, since it's one half of the system that controls the fuel and air mixture in a cv carb.
    #31
  12. Beemerboff

    Beemerboff Long timer

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    Amen , Amen
    #32
  13. AntonLargiader

    AntonLargiader Long timer

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    So Beemerboff, does this mean that a carb with a torn diaphragm is running too rich rather than too lean, since the slide is not lifting? This should be easy enough to test.
    #33
  14. Beemerboff

    Beemerboff Long timer

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    Please , read my posts, I did say that when the piston is opening slower that the butterfly is opening the mixture is richer, but if the rubber is torn so much that the piston is not moving that statement clearly does not apply.

    I have never had a torn diaphragm, SUs dont have them and I replace the Bing items as a service item - so I dont know just what happens, but I would imagine some fuel would be sucked in through the pin pricks, but there wouldnt be sufficient air flow to atomise it , so it wouldnt burn very well, if at all.

    So you could have a decent bit of fuel performing as if it wasnt there.

    Kinda like the brains of some of the posters on this forum.:D
    #34
  15. lkchris

    lkchris Albuquerque

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    The brains that count are those at BMW and Bing, and NOBODY on ANY forum can do it better.

    Look up your OE specifications and stick with them.
    #35
  16. AntonLargiader

    AntonLargiader Long timer

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    I dunno about that; it might very well apply. I can totally see your overall point. If the effect of lifting the needle only increases the orifice by let's say 20%, and you're getting 100% more velocity by virtue of the slide being lower, sounds like a recipe for more fuel rather than less.

    Doing the calcs wouldn't be that difficult but I think proving/disproving it experimentally would be easier. Thanks for the ideas.

    Because BMW has always gotten stuff like this exactly right. :rofl
    #36
  17. Hawk Medicine

    Hawk Medicine Coyote's Brother

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    I installed EUBMW's carb rebuild kits in my R90s carbs and ran with them for several+ years and had no problems or complaints. I also installed a set in my newly rebuilt R100s carbs and they appear to be working well also.

    All-in-all I find EUBMW to be a good bunch of folks to work with and for stuff like valves, springs and other hardware, you save a little $$$ and get the exact same parts as BMW provides.

    LAstly... What other parts supplier will ship you your parts before they receive payment? Thats service!!!
    #37
  18. coastranger

    coastranger Been here awhile

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    thanks for all the carb info, a true learning experience

    I too have used EUBMW for lots of things and have had no problem
    my last bike I used thier head gaskets, v.c gasket carb kits and pushrod seals . my current bike I have used thier carb kit on, no problems wiht any of it
    #38