Bing CV Carburetors, part 2
notes, TUNING...and overhaul items too.

© Copyright, 2011, R. Fleischer
bingcv-2.htm
4

This is an additional series of notes & hints on these carburetors as used on BMW Airhead motorcycles.

1.  Starting problems:  

 Starts OK, enrichener (choke) in use, then seems to run out of fuel, after perhaps 15-30 seconds:   Bad gasket at float bowl surrounding the enrichener jet well; OR.... partially clogged jet at the bottom of that well; OR...  gasket/screws problem; OR... wrongly assembled enrichener.    

Consistently hard to start??  Check the above.  Check that the gasket at the enrichener assembly is OK, not sucked inwards or pushed outwards.  If you might have installed the enrichener parts backwards, or mixed left and right parts, check for that. NOTE that some punch prick marks on the shafts have been seen factory-installed backwards!  That is, the punch prick marks are WRONGLY DONE.  See bingcv.htm article for a full discussion.   Check that the slides always fully return to the bottom, if not, add, if missing, to the top dome type carbs, this spring, above the slides:   13-11-1-335-324.

NOTE!....having an enrichener disc in backwards (whether by you mixing up left and right parts or the factory goofing on the punch prick mark or you installing the punch prick mark wrongly) can result in all sorts of various problems, not only for starting but in poor throttle performance at moderate throttle and rpm.   USUALLY the primary symptom is that the engine is very difficult to start, or starts only on one cylinder (neither, if both discs are in backwards).   One of the easy tests, once the engine is started and you can keep it running with a small amount of throttle, is, with the enrichener (choke) lever OFF:  LIFT, one carburetor at a time, its choke/enrichener outer cable at the carburetor.  If no bogging down of the engine (or other substantial change is heard), then that carburetor has an enrichener problem of some sort.  If the enrichener cover is loose you will have surging...see item 2. below.

One cylinder not firing and hard to start, then works OK as throttle is turned up, ...and similar things?   Besides a wrongly install enrichener, the SLIDE may not be returning....and you may end up polishing the slide and carb body and installing the very soft spring above the slide.  Was no such spring on the early carbs and even some later ones.  You can check the slide returning by looking at the rear of the carb after removing the plastic tube from the carb-to-aircleaner.  Fix that by installing springs 13-11-1-335-324 above the slides. See Bing's manual on various strengths of the spring.
 

2.  Surging at steady 30-50 mph:   The 4 screws that hold the enrichener assembly plate to the carburetor are known to loosen, and the gasket pulled inwards.  Early model /5 had no such gasket.   Disassemble the enrichener (do NOT mix up parts, especially left-to-right) and lightly lubricate things, do NOT plug the enrichener disc holes with grease!....use a new gasket, seal the screws with a TEENY drop of Loctite BLUE.

3.  Having to synchronize the throttle cables too often?  You may have old worn out cables.  Do NOT oil them except as a last resort before you get your new cables. They have a nylon or similar lining, so use only genuine BMW cables.  This is a good time to do the bi-yearly cleaning and re-greasing of the throttle assembly at the handlebars...and note that those gears must be lined up at the marks on their teeth during reassembly. 

HINT:  When doing your carburetors synchronization,  adjust the cable lengths at ~500 rpm above the idle rpm. This is the more critical point, where the butterflies are just barely off the idle stops.   Be sure you end up with a bit of cable's slack with the throttle off.  Up through /6 engines can usually be idled at about 800-900 rpm.  Later ones seem to do better at about 1025 pm.  If you adjust at nearer sea level, use perhaps 1050 or 1100, and then the rpm will not be too low at high altitudes.  These rpm values are OK for the stock single spark plug and also for the dual spark plugs conversion.  

4.  It is normal for the engine idle rpm to change with altitude.  This can be minimized by careful adjustment of rpm and idle mixture screws. 
You can do some slight compensating for altitude effects on idle rpm by properly adjusting the idle mixtures screws a wee bit, such that when you change altitudes the rpm effect is smaller.

5.  NEVER try to adjust the carburetors without FIRST knowing that the ignition timing is proper, & valve clearances are correct.  NEVER make the final adjustment of the carburetors without a thorough engine warmup. Garage warmup is NOT adequate.  Go for a minimum 10 mile ride!!!

6.  Float level:  The old method of adjusting the float level (the original dual single piece floats) is to have the carburetor upside down.  That is NOT needed.   A good method when the carburetor is already on the engine is to turn the gas off, remove ONE bowl, turn gas on, and lift the float slowly and VERY gently, until the gas JUST shuts off, and the top of the float should be parallel with the bottom of the carburetor casting.  DO double check this by now LOWERING the float until the gas just BARELY starts to flow.  The approximate same parallelness should exist.  You do this second step (or, do it as the first and only step) due to the spring loaded plunger at the lower end of some of the float needles; as well as some variations in the sloppiness of the float needle   In a rare case that you find the float needle plunger tit (most models have that) frozen up, fix that, usually with a new float needle. 

***If you experience surging or similar running out of fuel effects at cruising or high speed, and you open the gas cap with no help after 10 seconds, and fuel filters and fuel flow are checked to be OK, then you may have the adjustment of the floats for fuel level to be too low.

Actual bowl fuel level:  
Turn the gas off, remove float bowl, empty the fuel into the tank (look for globules of water, if so, toss the fuel someplace, like your cleaning tank), replace the bowl, turn gas on, allow bowl to fill, turn gas off, remove bowl quickly and squarely.   Measure the fuel depth from the center bottom to the fuel top, using something small...such as the tip end of a vernier caliper, or a thin, narrow, machinists steel rule. 
Do your best to have the bowl sitting flat and level on its base, and estimate the best you can with #1 eyeball, as there are meniscus errors, the exact height of the fuel from the bottom.  
For the 32 mm carburetors, use 24 mm fuel height.
For the 40 mm carburetors, use 28 mm fuel height.
If your find your gas level quite high, the one-piece white float unit may be bad.  Information on the floats is posted in a companion article on this website, BingCV.   Yes, that was a HYPERLINK.  ALSO refer to that article for information on removing/replacing the float.  The stock floats last about 30K, sometimes more.  ALWAYS replace the float needle at that time.

7.  After a LOT of miles, perhaps 60K, the slide needle and associated needle jet and the diaphragm will need replacing.   That is the time one usually cleans out the central jet assembly, uses a new O-ring in the central jet assembly and some silicone grease on it.  Cautions on this work are in bingcv.htm .  AVOID dipping carburetors into commercial carburetor dip-tank solutions, that will almost for sure eat up the butterfly shaft O-ring, necessitating its replacement....which involves removing those peened screws....and the need for careful MARKING, and then alignment and NOT reversing!) the butterfly.  Berryman B-12, the particular version called  'Choke and Carburetor Cleaner' works quite well to clean out passageways...use three times, with each spray allowed to sit a minute.    If you remove the butterfly shaft, DO put silicone grease on that butterfly/shaft O-ring.  It will last far longer and operate far smoother.  After mild grinding so you can remove the peened butterfly screws, be aware that peening new screws is not absolutely necessary, Loctite (blue) seems to work OK.  Again, be SURE the butterfly is installed correctly, and do not tighten the screws unless you are finger-holding the butterfly valve against the body, to be sure it is fully seated.   If you reverse the butterfly, it will NOT work properly near idle and off-idle.  The butterfly screws must not be tightened until finger pressure is on the butterfly in closed position, so it is centered in the carburetor.

NOTE!    from about 1986, Bing was shipping a rather soft needle and clip.   These wear fast, compared to earlier models.   They usually will not last the 60K miles I mention, above.

8.  Be very cautious about installing the needle jet assembly if the slide and needle are already in place.  The atomizer must stick up into the throat (it fits only one way, the two ends are NOT the same diameter), and you do NOT want the needle to hang up on an edge/corner of the atomizer.   AVOID any pressure, do NOT bend that needle!  NEVER force the central jet assembly...it should NOT require excessive force with wrench or fingers, to tighten it up.  Silicone grease its new O-ring before installing.

9.  The more common needle mounting method is such that the needles are adjusted to the desired 'notch', by rotating the needle 90 degrees and pulling down or pushing up during that rotation.  Some needles are held-in by a hidden clip, some by a screw.  The topmost notch on a needle is the #1 notch.  Most Airhead engines will use the second notch or the third.    I recommend that you check that both carburetors ACTUALLY MEASURE THE SAME for the needles, and write that value down.  Measure, with vernier calipers or similar, from base of slide to needle tip.  Try to hold accuracy to .015" or so on this.   This is a good way to ensure that you don't put the needles into the carburetors in the future by mistake on the wrong notch.  One notch is a BIG difference!    In fact, ONE notch on a carburetor needle is equivalent to 2-1/2 to 3 sizes of needle jet.

10.  If your bike tends to stumble and run out of power after a good uphill climb or a higher speed run down the highway (this could happen quicker with a very full tank), loosen the gas tank cap...if there is a whooshing noise and the bike runs well again after waiting 10 seconds or so, it is the tank venting that is clogged.  Similar problems with fuel flow can come from clogged in-tank or in-petcock filters, or clogged aftermarket filters.   As noted earlier, too low a fuel level can also cause this sort of problem.

11.  Most petcock models (there are at least 5 models), if they get stiff, can be taken apart, cleaned and silicone-lubed.   Earlier versions of two petcock styles had a tall screen that stuck up inside the tank, that screen used a separate reddish-brown cambric washer.  Remove washer and screen and replace with the later one-piece bonded screen/washer.  It not only screens particles better, but seals better. Some petcock models have a screen built into the lower outlet...remove and clean it regularly.    Remove and clean and dry out the tank every year too.  The original /5 'bullet' petcock is rebuildable, IF..IF...you can get it apart, which really will mess-up the appearance and metal.  It is nice to have an original bike appearance, but the much later petcocks work find, and are easily rebuildable...often only cleaning and a tad of silicone grease is all that is required.

12.  The band clamp that holds the curved plastic intake tube that fits between the carburetor intake and the aircleaner housing should be installed with the screw portion of the clamp ON TOP...or at least not at the bottom, at the carburetor end.  This will help avoid breather oil dripping on your foot.  This is applicable to the /5 models, but check the plastic pipe that connects the aircleaner to the carburetor, and if it has slots at the bottom of the end that fits over the carburetor inlet, then follow this advice for sure.

13.  FOR SURE use a faint dab of anti-seize on the dome top screws...threads AND the taper under the head... before assembly. Tighten them evenly.

14.  If the carburetors are being worked on off the bike, you might consider doing some even-pressured figure eights on a piece of glass with upside down 220grit sandpaper, and then 360 grit sandpaper, on the carburetor body, top portion. Just enough to clean it up, and a tad more. It will help stop those ugly gas stains, and might solve some strange problems you had been having. 

15.  Except for the flat top models, the dome should be checked for "leaking knock-out plugs"; they are located at the very top.  If they are leaking in the slightest, seal them.  You can test for leaking by putting gasoline into the top if it is removed and upside down.   You can try carefully re-staking the plug if you want to, or just cleaning the edges carefully, perhaps with a teeny tool and teeny bit of sandpaper....and using clear epoxy or other sealant.  I usually just use a dental pick to clean the edges before using epoxy.   Some folks also epoxy a nice BMW Roundel on the tops....see my Roundels page for a full listing, with sizes, and one or two DO fit those small top areas.

16.  Diaphragms:  Here is a link to information on earliest Triumph Spitfire.....you will have to wade through menus to the fuel system...may still operate:  http://www.spitbits.com.    Stromberg CD150 ""supposedly"" uses the same diaphragm (B20924??) as the Bing 32mm.  For 40 mm, it was CD175.  See also 1970's to 1980's Volvo.  My personal opinion is that the Stromberg diaphragms are not the same in flexibility, and vary otherwise too...and....are for emergency use only....and you could, if yours gets a rip or tear, simply glue it, or use a glued-piece of condom, or whatever.  It is my belief that the Stromberg diaphragms vary greatly with supplier, and if too stiff, you will have LOUSY power availability, and likely an inability to rev high enough.   There is a rumor, that I do not know the truth for, that Bing USA is selling the Stromberg diaphragms under their own Bing label.   My advice remains exactly the same as it has been for years...do NOT buy rubber parts nor diaphragms from Bing USA.  Do not purchase any carburetor parts from BINGUSA.....UNLESS the parts you need are NOT sold by BMW!   Use genuine BMW parts.  STRANGELY, your BMW dealership usually sells the carb parts cheaper than Bing USA.
The 32 mm overhaul kit from BMW is 13-11-1-258-051
The 40 mm overhaul kit from BMW is 13-11-1-336-902.

The diaphragm has a locating tab, that lines up with the similar recess in the carburetor body. Do not allow the diaphragm to slip out of position during assembly. The slide has a couple of holes in the bottom, they face the engine. Do NOT allow the slide to get nicked. Don't leave out the spring ...if your carburetor has one...on the top of the slide assembly.     
 

Although this article is for the Bing CV carburetors, Bing did supply a 26 mm slide carburetor for some early Airheads.
The kit for them is 13-11-1-258-050.

NOTE:  Do NOT!!!...NOT!!!!...use anything but BMW or Bing Dealership's furnished BMW diaphragms! You may run across articles that say that you can substitute Zenith Stromberg diaphragms, used on some Volvo's, ETC., for certain Bing diaphragms.   You also may find that if you do this, they may work OK, BUT MAY work marginally.  There are differences, in material/thickness that do NOT seem consistent.   The Stromberg CD150 diaphragm does physically fit the 32 mm Bing CV; and the CD175 does physically fit the 40 mm Bing CV.  DO NOT USE THESE.

 With some Airhead carburetors; BMW does not offer just the diaphragm, and wants you to purchase the slide with the diaphragm attached.   This may occur with the models where the diaphragm is attached by a pressed-on nylon ring.  Purchase the diaphragm from BINGUSA.   You CAN pry the ring off (DO USE HOT WATER to soften and expand the ring to avoid injuring the ring why removing and replacing).  

I highly recommend you do NOT get carburetor rubber parts from BINGUSA; get them only from a BMW dealership, they will be correct color and type of rubber, etc.

BMW dealers prices are generally CHEAPER than Bing prices.....and you can NOT depend on Bing USA parts being of the same quality as the BMW sold parts.   Be careful! .....and do NOT use Stromberg diaphragms!

17.  Carb Top Screws:
((MIND the information given earlier, to put a trace of antiseize compound on the screw threads AND the taper))

BMW supplies a screwdriver 71-11-1-103-086  in the on-bike tool kit.   That screwdriver blade pulls out of the plastic handle, and is reversible in the handle...and thusly you have both a Phillips and a standard screwdriver.  The Phillips tip is NOT a Posi Drive nor is it a Reed & Prince tip.  There are better choices than the BMW screwdriver.  Better choices are stronger, but it is not always easy to find a screwdriver that is small, and has both Phillips and slot ends, and fits your tools bag for the on-bike kit.  Some folks purchase one of the SnapOn types, with assorted hardened magnetic tips that fit inside the handle.   If you have the room in your tool tray on the bike... and budget... the Snap-On is a QUALITY tool, that NEVER wears out the tips.  Be sure you have the proper size tips on whatever tool you purchase...many come only with a relatively narrow single blade size tip.  The proper Phillips size is #2; IF you have Phillips screws...and you WILL at other places on your Airhead.     BMW carb top "Phillips" screws may not really be Phillips screws, they just might look that way at a first glance. They could be Phillips, OR could be 'Posi' type.   The ancient aircraft tip called Reed & Prince works nicely-enough on the Posi screws....as, of course, does the real Posi (better!).   The Posi tip is EXCELLENT for REMOVING the Phillips type, if the Phillips is very tight.   Phillips screws were DESIGNED to SLIP after a certain torque level is reached!  Install a Phillips type with a Phillips screwdriver.   If the screws are frozen, you can try a variety of ideas, see my other carb articles.  This includes valve grinding compound for a better grip, a metal block underneath and, especially, an Impakt Driver, with the carb body SUPPORTED!  Interchangeable-tip type tools are available from a variety of sources, including Snap-On.  YES, the Posi IS available.    The only critical place on our Airheads for this type of screw is the screws used on the top of the Bing CV carburetors.  Some have installed Allen head screws there.  They are OK, but don't overtighten, as many of these have a very small allen size and can round-out more easily. I DISLIKE Allen screws there; particularly if not the tapered type (under the head).    Some carbs had common single slot screws.   BMW and Bing HAVE shipped both Posi & Phillips screws......be sure that your 'screwdriver' fits them, and if needed then do obtain a Posi #2 or Reed and Prince screwdriver #2 or tips.    Remove the screws one at a time, coat the threads...and taper...with antiseize....and replace the screws only moderately tight.  BMW's red plastic-handled screwdriver that is in the BMW on-bike tool kit, the one with the reversible insert, probably Heyco Germany brand, is NOT THE CORRECT TOOL for the Posi Drive screws in the carburetor tops!!!....this tool is, instead, a common Phillips!!   The Posi screws generally (not always) have some radiating lines to indicate they are not Phillips type.
 
 Here are photos of the Posi-Drive screw, and the screwdriver tip.  Note the differences from a Phillips or a Reed & Prince.  Notice, for instance, the nearly flat bottom in the screw and the associated tip of the PosiDrive screwdriver.....note that a Reed & Prince tip is SHARP.   Note also that the PosiDrive has partial 'splines' in between each regular size 'spline'.      PosiDrive's are NOT designed to slip at the low torque that a Phillips screw IS designed to slip at.
    

It is not uncommon to have the top screws on the Bing CV carbs seize up.   This will not happen if you use a faint bit of antiseize compound when you reinstall the screws.....on the threads and taper area.

a.   Be sure that you use some sort of pointy thing to clean the cavity of the screw before trying to remove it.
b.   Be SURE to use the proper size of the proper screwdriver.  Use of a too-large one will damage the screw, or cause the screwdriver to slip out.  PosiDrive tips for changeable tip screwdrivers ARE available.
c.   Use a wee tad of valve-grinding (gritty stuff) compound can often help if using a Phillips screwdriver, to grab the screw better.
d.  With single slot screws you should have a screwdriver that actually fits the groove without a lot of play, and fits the width of the screw fully, but not wider.
e.   Do NOT use an impact driver if YOU do NOT support the carburetor boss area. Impact drivers are very handy hand tools for loosening fittings.  They were a MUST on the old Japanese bikes with those 'Phillips' screws.   To use one safely, you must either have the carburetor off the bike with the boss supported on the work bench, or support the boss area somehow and KNOW how much force is safe!  

#18, below, describes this tool

18.  You won't find a lot of need for this tool, but when you need it, and it is a tool to have at home, you will cuss without it.  This is a tool called an impact driver.  The brand I have is named IMPAKDRIVER, made by the H.K. Porter company.  This neat gadget is hit with a hammer, WHILE you hold CW (tightening) or CCW (loosening)  pressure on the tool.  The hammer blow pushes the tool tip (lots of tips available) into the screw, and the internal CAM of the tool causes rotation.  This tool was a MUST with old Japanese bikes, and with a few bits, you will find it valuable.   Use this one, carefully, on such as carburetor top screws (don't break the carburetor tab ...support the area!), and other such places.  Good for small nuts, not just screws.   I get this tool out BEFORE I round out a head.  More information in the TOOLS article.

19.   Leaking from float bowls, etc.??....There are numerous reasons this can happen, such as a bad float needle (or float needle seat,  more rare).  However, there is an obscure reason, that was first found on, I THINK, the 1988 R100 models with 32 mm carburetors.  Vibration....at high speeds usually...can cause the float bowl to overflow.  BMW found the cause, and issued a Bulletin.  It is, believe it or not, due to the wrong type of rubber hose material, between carburetor and the cylinder head.   The proper hose is 13-72-1-254-654.  DO NOT use the -338-360 hose.....supposedly used on the R100GS/PD, but may be on your bike, and not just the 1988 models.  The soft -360 was used by BMW to address FOAMING in the float bowl.  USE the -654, and check the fuel level, and run the fuel level slightly on the high side of specifications.

20.   FLOAT NEEDLE SEATS:   It is rare, but once in a great while there is a need to replace a float needle SEAT.   You may hear that they are not replaceable, or that only Bing can do it.  NOT SO.     Some folks have drilled them and used an EZ-Out to remove the old one.   But, these seats are rather strongly pressed into place if you try to remove them withOUT heating the carburetor; and even then that may not release them.   Here is a method, you can improvise your own:
a.  Tap the seat with a 7 mm x 1 mm tap.   You could use SAE taps too...with American threads.
b.  Use a screw of 7 x 1 mm thread, or American threads if you used that type...maybe 50 mm (2") or so long.   Use some sort of bushing, perhaps about 8+ mm in INSIDE diameter, and about 14mm or so OUTSIDE diameter.   This bushing should be around 25 mm long.  NOTHING critical here except that the OD is important so the bushing fits the needle seat carburetor area, not the needle seat.   You can now use that screw and the bushing (and maybe a washer) to DRAW OUT the old needle seat.    HEATING may well help.....as I THINK the carburetor body material expands faster than the needle seat material.   
c. Use heat on the carburetor body when installing the new seat....which can be carefully tapped into place with a flat tool.
d.  You need not use metric taps and screws, use any screw thread you like, that fits properly.

21.  Some very late model R100GS, POSSIBLY others, and probably only 40 mm (??) Euro shipped models (?), may be found with a groove in the upper area of the slide, with an O-ring at that point.   The purpose is to help eliminate a rattle these sometimes had.  It isn't necessary to retrofit YOUR bike with these, unless you have a bad rattle at idle (often on right side), and dislike the rattle.

22.   When assembling the enrichener valve, etc. (see bingcv.htm for all the details), I recommend a VERY FAINT smear of a NON-RTV type gasket sealant, on the outer surface, carb surface, or just both sides of the gasket.  Gasgacinch or Permatex form-a-gasket non-hardening, or similar, is fine.   The reason is not to prevent leaks directly, but to prevent the gasket from sucking-inwards, which then DOES cause leaks.

23.  HIGH IDLE RPM AFTER WARMUP:
There are several TESTS for proving that the ATU is at fault (and not, say, a vacuum leak at the intake rubber hoses, no free play in the throttle cables; or a mal-adjusted idle mixture screw).  The problem occurs almost only with the CANISTER models of ignition.
In some instances, just turning off the engine and restarting it after it was already hot and exhibiting the very high idle, is enough to reset a stuck ATU, so try that.  If that now shows a normal idle, it is likely a ATU problem.
If not get a friend's help.  First take the bike for a ride, and if the idle rpm went quite high after a FULL WARMUP of the ENGINE CASE, then pull the bike up to a nice big object, like a brick building.  You could also just use the front brake.
With the bike in gear, let out the clutch very slowly, loading the engine and allowing the engine to slow down to about normal idle, perhaps 900-1100 rpm.   Have a friend use a timing light, triggered from the left spark plug, point the light at the timing hole. If the timing is well-advanced, then the ATU IS THE PROBLEM.  Prove it by pulling in the clutch...you have a high idle rpm again, right?

24.  This was reported to me by Tom Cutter.   Tom says that this modification, installing a longer and softer spring on the slide, will smooth the idle transition at the 1/8th to 1/2 throttle movement.  Tom said that the shorter, earlier spring, used on the 40 mm Bing CV carbs, was 13-11-1-335-324.    He installs the longer softer spring 13-11-1-338-134, as used on the 1988-1995 R100 models using the 32 mm carbs (these springs, per Tom, are used on the EURO R100GS 40 mm carbs).

25.

Tuning Bing Carburetors....a brief outline...covering changing jetting and high altitude riding

Revisions:
final update and release to website 12/08/2002 5:35 p.m., PDT., includes through item #10 only.

03/04/2007:  all prior updates checked; minor clarifications including #13 added, and re-released to Internet.
12/01/2007:  Update entire article; rearranging and simplifying in some areas, expanding in others. Add information
                    on the soft needles.
11/27/2009:  Add bottom portion on tuning.  Fix some errors (wrong hyperlink), and other minor things.
04/20/2010:  minor updating and clarifications
04/25/2010:  add photos of Posi screw and tip; and later in the day, updated the article for clarity and appearance (NO changes to content details).
04/30/2010:  remove, then replace, the two above photos, as the link did not work, even though appeared-to.
05/16/2011:  Clean up article a bit, clarify some details.  No important changes.
06/07/2011:  Add #23
06/08/2011:  Add #24
08/15/2011:  Expand #16 and edit

© Copyright, 2011, R. Fleischer

Return to Technical Articles List Page

Return to HomePage