"Beefier" Airhead Diaphragm

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

  1. snoop

    snoop Sven from Road Rash

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    From the sticky dark secrets thread.


    Does anybody have experience with these, I ripped mine trying to get it out and need a replacement. It looks like bmw doesnt sell these seperate anymore.
    #1
  2. Stagehand

    Stagehand Imperfectionist

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    you can get them straight from Bing, no problems.

    Why the hell is one 20 bucks and the other one $3? :lol3
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  3. TaterHarry

    TaterHarry Redneck Emeritus

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    Order them from Bing if you must. That said I think the dealer still carries a full rebuild kit either with or without the diaphragms, so you might go that route. The "beefier" Stromberg diaphragms are also stiffer, and are not recommended except in an emergency IIRC.
    #3
  4. Stagehand

    Stagehand Imperfectionist

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    just saw eubmw's got whole Bing carb kits for under $30 :deal
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  5. Beemerboff

    Beemerboff Long timer

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    A little stiffer isnt necessary a bad thing .

    One of the true secrets to tuning CV cards is selecting the correct slide spring / needle combo

    Bing has a choice of one spring and around two needles, so no problems with selection here , but SU has a choice of eight springs and 600 needles, so you have lots to play with.

    A stiffer spring/ diaphram will richen the mixture in transition , so you could probably drop the needle a notch , and get better throttle response and gas mileage at the same time.
    #5
  6. jtwind

    jtwind Wisconsin Airhead

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    Maybe a heads up here. I had a buddy buy some parts from these guys. Most of what he bought was not Bing or BMW stuff. It was aftermarket crap. He installed pushrod seals from these guys and all 4 split from top to bottom shortly after being torqued up and running. Can't say for sure about all their parts or carb parts, but asking if they are BMW or Bing parts maybe a good idea. Trying to save a few cents for some of this stuff can backfire. Some of the stuff is cheap for reason.
    #6
  7. Stagehand

    Stagehand Imperfectionist

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    excellent point. :deal

    well taken. :cool2
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  8. Solo Lobo

    Solo Lobo airhead or nothing

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    I ran these carb rebuild kits in my old R100RS, and then in my old R100GS since 05/2006 with no issues and have a set of rebuild kits on the bench from them for my R80G/S... I do keep the diaphrams I pull out as emergency spares on the bike to rescue myself (or any other 32mm bing carb users...)
    #8
  9. soyanarchisto

    soyanarchisto Long timer

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    I bought carb rebuild supplies from eubmw and had no issues whatsoever. Still going strong.
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  10. bikecat

    bikecat Long timer

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    Came across this thread as I am starting a carb rebuild.

    Any further reviews on the EUBMW rebuild kit? I realised that I bought them about a year back when the bike was still very much work in progress.

    Will get the jets etc from Motobins, I reckon. Pound is low now and might as well take advantage of that.

    Cheers
    #10
  11. squish

    squish Out of the office.

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    Wait a second, how would this work?
    A stiffer spring and or diaphram would be resistant to lift under increased intake vacuum. (When you give a CV carb more "gas" what you really are doing is giving it more air. your opening a butterfly that allows more air into the intake tract which causes the slide and needle to lift.)

    If the slide's not lifting, it's not flowing more fuel (since the needle is further into the needle jet effectively blocking more gas from coming up the emulsion tube.

    Seems to me what happens is it makes the engine a slight bit more sluggish to respond to intake vacuum. Which in turn makes the rider dial in more throttle for a given increase in engine speed.
    #11
  12. One Less Harley

    One Less Harley OH.THAT'S GONNA HURT

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    I agree w/ squish on this.

    and when does running richer mean BETTER mileage??????
    #12
  13. Beemerboff

    Beemerboff Long timer

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    Read my post again .

    If the butterfly opens faster than the slide there is more air flowing in the same space, faster flow therefore more suction at the needle jet, more fuel sucked up and a richer mixture in transition, where it is needed.

    Faster flow is beneficial too - just look at the problems fuel injected bikes have at low flow - they just dont have to flow speed to fuel properly.

    At steady speed , no movement of the butterfly, the stiffer spring / diaphram will still hold the slide a little lower, richer, but the difference is not as marked as during transition.
    As most carbs run plenty rich at a steady speed they can be leaned off here to benifit, and if this can be done without a detrimential effect during transition then higher mileage is possible.

    Fitting a stiffer slide spring and lowering the needle is one way of doing this - there are a choice of 8 springs availible for SU carbs and the selection of the correct one is one of the secrets to tuning a CV carb.

    But you wont read about it anywhere but here, I got it verbally from the guy in carb specialists Midel in Sydney when I was working on a good set up for 2" SUs on a Datsun Z, and I have never seen it in print anywhere, including in the books on tuning SU carbs.

    But then I have my own views on a lot of things, so ignore them if you wish - I am used to being told I am wrong and I wont mind one bit.
    #13
  14. One Less Harley

    One Less Harley OH.THAT'S GONNA HURT

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    At steady speed , no movement of the butterfly, the stiffer spring / diaphram will still hold the slide a little lower, richer, but the difference is not as marked as during transition.
    I'm more than willing to learn...but a lower needle means leaner...doesn't it??? Unless I'm missing something??? Just doesn't seem right, but I'm rather heavy handed on SU's, be that Hitachi's, ZS's or Bings. So, your saying more fuel due to suction even with a lower needle..hum, never thought of it that way, better atomization too
    #14
  15. squish

    squish Out of the office.

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    essh...
    Ok here's the deal the lower the slide is in the carb the less air and fuel is traveling through the Carb.
    Basic quick primer on CV carbs..
    The butterfly controls the amount of air that CAN come into the carb.

    The slide controls the amount of air and fuel that go into the combustion chamber.

    See the needle(also Jet Needle) is connected to and moves in conjunction with the slide.
    If that slide doesn't lift there's going to be less fuel, not more.

    That is because the needle sits in a device called the emulsion tube. which at the other end sits in the float bowl into the fuel that's there, the tube is capped off by a device that's called the Needle Jet (also called the Main jet or just the "main"). This is the first part of the fuel metering system.
    The second part is that needle, it's got a taper on it. and it's controlled by the slide, as the slide moves up. the needle is pulled up. this allows more fuel to flow up the emulsion tube and be "sucked" into the flow of air that is heading into the combustion chamber.
    Intake velocity's not going to matter that much when the needle is working like a plug in the emulsion tube.

    Second, at least here in the US
    Carbs are set very lean at idle and lean in the mid range and almost rich on the top.
    Mid throttle settings, that is the slide has just lifted and the fuel is almost all being metered by the jet needle and needle jet.
    are most often lean or maybe just a little past lean toward rich.

    As for fuel injection bikes.
    One of the biggest areas where FI controlled engines work is at slower engine speeds or when an engine is cold,
    The can produce the correct amount, and often with better fuel atomization then a carb can.

    The biggest drawback to lots of FI systems is how abrupt the fueling is compared to the rather unsophisticated CV carb system is.
    Depending on how the FI system is controlled this can lead to surging
    Like early BMW oilheads.
    #15
  16. Beemerboff

    Beemerboff Long timer

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    Speed , and suction , is most everything.

    These pictures are OK, but they just dont mention speed, and therefore suction and flow.

    And understanding that is the key to understanding and tuning Constant Velocity carbs.

    If you read most tests of FI bikes the tester notes problems with fueling at low revs, but if it was simple mixture problem it would be easily sorted.

    The problem is maintaining air flow speed without a CV device somewhere in the system - Hondas new VFR 1200 has one for that very reason..

    When I fitted SUs to a Z I was building a torquer 2850 cc P90 motor , and as the 1 3/4" Hitachis only flow enough for 175/180 hp it was hitting the wall before 5000 revs.

    Triple 38mm Webers are the track set up, but I wanted something more streetable, and I already had the 2" SUs, which are good for around 250 hp aqnd would therefore do the job with a bit to spare.

    Starting from the Healy 3000 settings it wasnt a big job the get them close in warm weather, but fitted to a unheated manifold and with pod filters, tuning for cold running and the changing seasons was a bit harder.
    As was final fine tuning.

    Both problems were solved by stiffer springs , a good stiff set for the freezing winter mornings, and progressively softer as the seasons warmed up, but still quite stiff.

    So I am as certain as I can be of the effect of stronger springs.

    FWIW there is more than one spring available for BMW Bings - the part number for the spring fitted to the later bikes is different, and the spring is slightly stiffer.

    Some guys on the English Owners Club forum did a comparison , and found the later, stiffer , spring to perform better , but I have never tried them myself as I still have a good selection of SU ones.
    #16
  17. Rucksta

    Rucksta SS Blowhard

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    Venturi
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  18. BMWzenrider

    BMWzenrider The Road Scholar

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    CV, as in CONSTANT VELOCITY of airflow through the slide area, is the basic goal of the Bing (and similar) carb designs.

    The idea is to maintain the same velocity of air through the slide to provide the same venturi suction pressure drawing fuel up at all times.
    Improves atomization, makes metering more predictable...

    When the engine RPM increases faster than the slide rises, the velocity through the slide area increases, which increases venturi pressure/vacuum. That will draw more fuel up for the same size opening as a lower velocity.

    A stiffer spring (or diaphram) will slow down the rate that the slide rises, causing a temporary increase in velocity through the metering section (slide) of the carburator while accelerating. That will result in more fuel (richer mix) being drawn for that timeframe at the same needle setting.
    As the slide catches up as steady engine speed, the velocity in the metering section reduces, and the fuel metering (mixture) goes back to the same as with a softer (faster) spring.

    What is happening at the butterfly is irrelevant to this analysis. All Beemerboff is talking about is what happens at the needle during the transition phase while the slide is rising.

    There IS a temporary enrichening of the mixture due to the slower response of the slide with a stiffer spring or diaphram.
    That allows you to run a leaner needle or main jet setting for improved fuel economy at steady speed riding, while still pumping more fuel when accelerating to help performance.

    -----

    Yes, it may seem counter-intuitive at first, but it does work that way...
    #18
  19. squish

    squish Out of the office.

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    This type of carb is called, depending on your country a
    CD Constant Depression, or
    CV Constant Vacuum or more colloquial called a Constant Velocity carb.

    The name derives from the slide lifting due to intact tract vacuum.

    The more proper term is not "slide area" but rather throat, *choke or venturi. (*choke here is not in reference to the cold start device but rather the passage way where the fuel and air mix)
    Air doesn't not flow through the slide. The slide lifts as air moves through the throat of the carb.

    The intake tract vacuum raises the slide. if the slide doesn't lift, the RPM do not increase increase.
    Except at the very bottom of the throttle movement, when there is very little intake vacuum. then the fuel is metered by the idle circuit.

    The slide is the device that moves up and down, it's connected at the top to the diaphram, and at its bottom is the jet needle (or simply needle)

    Here's the rub. The air moving through the venturi will pull fuel up the emulsion tube. But, sitting in the emulsion tube, hanging off the bottom of the slide is the needle. This tapered device is controlling the amount of fuel making it's way up the emulsion tube regardless of venturi velocity.
    That's what it's there to do.

    A heavier spring, heavier slide or stiffer diaprham will slow down the lift of the slide. this will constrict the rate of fuel that is pulled up.

    The butterfly (throttle) is THE valve that controls the amount of air flowing into the system.
    If it's closed there is very little air flowing, If there is no air flowing the slide is not going to lift.
    The only way the slide can lift is by the vacuum created by the engine RPM increasing.

    So let me get this straight...
    The temporary "rich" situation of the mixture due to a slower slide lifting allows for a leaner needle or needle jet setting for steady speed riding...

    A situation where the slide doesn't rise since it already rose to the steady engine speed position. :huh

    Sorry, Don't believe it for a nanosecond,
    But hey it's not the first time I was wrong. So I went searching
    I came up with nothing.
    none of my books on CV carbs or engine tuning mention any thing of this sort.

    Now I'm not saying that changing the slide weight doesn't change the character of how the engine responds. But making the slide heavier doesn't make the mixture richer.
    #19
  20. Rucksta

    Rucksta SS Blowhard

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    The butterfly (throttle) is THE valve that controls the amount of air that can flow into the system.

    An open butterfly does not cause air to flow, but a closed butterfly will restrict the airflow.
    #20