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EARLY R75/5 BING CV CARBURETORS; problems, including idle problems.

Copyright, 2013, R. Fleischer

While it may appear that the information in this posting does NOT apply to the later carburetors, except for information as to how the /9 and /10 carburetors are different from these /3 and /4, that is not so!   The information in this article should be read by anyone that works on later model CV carbs some things are good to know....such as about the springs that might be added, and many other things.

Special note:   Before working on the R75/5 carburetors to cure what APPEARS to be carburetor problems, such as with high or irregular idle rpm or inability to properly adjust the carburetors, it is important that the valves be set correctly, and the ignition timing be correct.  Set idle rpm to 900-950 with stock weight flywheel...1025 with machined lighter flywheel, maximum advance at the F dot on the flywheel, occurs ~3000 rpm, NOT 2000!   SLOW idle timing mark approximately the "S" mark.   If the Automatic Timing Unit, ATU, has the  early soft springs (2000 rpm is showing F dot), they should be changed to the later springs, for use with today's lousy gasoline's.  If you have 91+ octane available, you can consider using the early ATU....well, early springs.  However, they do allow some small ignition advance from ~800 rpm.  Since, in MY opinion, the idle rpm should be within the range of ~900-1050, that means some advance will be had even at idle.   That complicates things, and is further complicated if the advance springs have stretched-out some, and worse if they have stretched enough to allow SLOP in the advance weights (easily seen, engine OFF).    The difference in acceleration/performance is so slight between the "2000" rpm springs and the "3000" rpm springs, and considering the other things......that I think it BEST to use the 3000 springs, no matter the gasoline, etc.   I do not recommend shaving the advance weights to obtain the higher rpm, but that IS OK.


This article deals with problems on adjusting and operating and overhauling the early version of the CV carburetors used on the R75/5; getting the bike to start and to idle correctly, ETC., as best possible.  At the end of the article in the Addendum is some information on converting choke cables from the iffy stranded cables, etc. 

As many of  you know, if you have followed the Airheads LIST for some time, I wrenched during 2001 on a borrowed early (12/1970) SWB R75/5 with the original carburetors, that had been NOT so nicely overhauled in the recent past by Bing themselves. Putting it mildly, Bing miss-drilled two holes, and generally made a mess. Even when originally delivered brand-new, these early carburetors had some badly made (and designed) parts, besides the problems with an early design that was NOT well sorted-out.   This was not the first time I had 'played' with the R75/5 CV carbs...I'd a lot of experience with them, back as far as the seventies.   But, this particular set of carbs was a real mess.

One of the annoying things about this /5 that I strove to fix was a goodly tendency for wandering idle and also sometimes having to use the choke to start the engine when the engine was already hot from riding. Trying to get a solid and stable idling adjustment on these very early CV carburetors is often difficult. Most folks give up, and install carburetors from the R75/7, not a bad idea actually, if obtained reasonably priced. I had not played with this series of carburetor, the /3 and /4 as originally shipped on the R75/5, for many years, and had the time to fiddle, quite a lot of fiddling.  While the owner was willing to purchase the /7 carburetors, I wanted to keep the bike as stock as I decided to see how frustrated I could get!   I am ornery, dogged, and nerdy [among my better attributes], and I felt I could make these carburetors perform better, but not as good as the later models. This goal was accomplished. 

These early model Bing 64/32/3 and 64/32/4 carburetors were a bit crude. The idle passageways in the carburetor were a poorly designed simple single stage type and were somewhat abrupt in operation as the throttle is barely opened from idle stops. That is the primary reason that the butterfly's had to be installed absolutely correctly. Even on much later models, the positioning of the butterfly is important. 

On these early /5 CV carbs, Bing put some idle holes in a slightly wrong place.  In my estimation anyway.

All of this often leads to easy, no-choke (enrichener) cool weather starting...which SEEMS like a nice thing at first glance; but off-idle response can be poor...and...well, read the rest of this article.

Note also that the choke (enrichener) was cruder in its internal design than later models.

O-ring sealing was not as extensive. The original early diaphragms were thinner, replacements are thicker. There were no vacuum take-off ports (Bing does make a kit, and you CAN drill the carburetors and install their kit, or make your own).

Some materials in the carburetors were absolutely not conducive to smooth and consistent operation...the slide guide pilot was steel, later models are brass and hard chrome plated. The choke (enrichener) was a sort-of one-stage type, poorly sealed as it had no gasket.

Originally the 2nd needle position was used, and the needle jet was a quite rich type. The slide itself was the earliest, and primitive. 

There were a number of bulletins (Service Information...or SI) from BMW on these early carburetors. The stamped number on the ESPECIALLY TROUBLESOME carburetors is 64/32/3 and 64/32/4.


Specific details:

Beginning with engine number 2 972 061 for the R75/5, they changed the carburetor construction. The slide guide pilot, formerly steel, was now brass, and hard chrome plated. The diaphragms were increased to .016" thickness. Never an official first..., but sure not hard to figure out why the changes!

After serial 2 977 320, the carburetor model number was changed and it ended in  /9 and /10.  Along the way were more changes.

The earliest BMW bulletin has a lot of information, regarding measuring the slide needle below the slide (1.555" means third notch); information on slotting the air-tubes; keeping cylinder timing variation to under 1 degree.  Yeh, RIIIGHT....this also meant playing with the cam nose end....dangerous except for experienced wrenches...although BMW ALSO said to STONE the automatic advance cam so timing did not vary between cylinders by more than 1...laughable, as cam tip run-out is typically more;...BMW never mentioned the brass hammer bit on the CAMSHAFT nose;...careful here folks!.  Not going to get into all this right here.  I am sure most of you know that a little timing chain (well, timing SPROCKETS) wear makes this even more laughable.

Another bulletin came out dealing with the diaphragm installation, evenly torqueing diaphragm screws, adjustments, float levels (.767-.787 inch of fuel)....tuning, etc. Some of this stuff is interesting to folks like me...and some, like the needle and fuel measurement, are hardly found ANYplace else except in old files like mine. 

There were some little known INternal changes in bores, passageways, etc. The early R75 carbs were STILL having problems, AFTER factory bulletins and work by BMW prior to dealer delivery, PLUS dealer work. Bing did all sorts of things not commonly known. Bing sealed the mixture tube, finally by an O-ring; the idle nozzle was changed, bypass bore size changed, they even changed the float needle spring, made some changes in passageway sizes, and there was a change in the ventilation of the "starter cup" from outside to inside, as they put it...or; mixture bore in the rotary valve.  Confused yet?

It was at this later time, well into many changes by Bing, that the enrichener was modified and now had 3 different sizes of fuel feed bores. 

****It was in November of 1971 that BMW first ...and almost lastly....recommended the use of OPENING the throttle during very cold starts. We all know that this can be a necessity depending on the bike, temperature (does NOT have to be really very cold), and how the carburetors are adjusted.   That is still true for later models, but BMW has dropped the recommendation of an opening throttle.  SHAME.

Problems folks have with the /3 and /4 carburetors includes not only what has been mentioned, but also a slow return to idle, poor throttle feel/performance, surging in the mid-range, but especially an irregular idle. In some cases, the idle STOP screw will allow the butterfly to close quite far, and as it does so almost completely, the idle speed goes mysteriously UP!! In fact, if you have one of the particularly bad carburetors, you may have a problem getting that cylinder to idle slow enough! I won't get into just why, as it is not terribly important here. Be SURE that if the butterfly's were removed, no matter by who, that includes BING!, that they are installed correctly!  They must fit the carburetor bore properly, particularly at the bottom and must not be installed backwards.  Later in this article are photos of the butterfly done correctly, and INcorrectly.

The need for critical adjustment of idle mixture, idle speed, and interaction of those; and, the effect of not quite perfect cable synchronization, is PRONOUNCED on these early carburetors, as are altitude effects on idle speed.  Everything has to be JUST RIGHT for anything even approaching a decent synchronization and stable operation. Even then, they are not as nice as the later models, and perhaps cannot be made quite so, but they CAN be made to be QUITE acceptable.  MANY have NO problems, even with large mileages on them!!...and THAT is due to the inconsistencies of the manufacturing. 

This is NOT a "how to overhaul a Bing CV" article. A true overhaul is not just a thorough cleaning; but, includes full cleaning and new float, float needle, slide needle, slide needle jet, diaphragm, and probably shaft O-ring and other O-rings (some carburetors, the very earliest R75/5, had some O-rings missing, compared to the later carburetors, even with the same model number). I will assume that your carburetors are basically in good condition, parts-wise, including NON-leaking dome inserts!!   I will also assume you know how to spray brake cleaner on the rubber adaptors to the cylinder heads to see if they are leaking (idle speed changes); and know how to adjust the carburetors.  If not, read my other carburetor articles!!

 BMW made a number of modifications to these carburetors themselves; or, dealers did, and those modified carburetors are SUPPOSED to be stamped with a letter C on the knock-out plug on the dome cover. Another identifying point is that the later carburetors HAD a GASKET at the enrichener cover plate. When BMW modified EXISTING carburetors, they changed the dome cover, slide assembly, and the enricheners too, and the enricheners are handed (left, right). If you have dome insert carburetors, SEAL THE DOME, as I have described in my articles.

NOTE that Bing's own carburetor manual is NOT correct on a few minor details, such as the enrichener parts. 

Quite frankly, if you cannot get these /3 and /4 carburetors to work right, after some playing, perhaps you should plant flowers in them.  BUT, do keep in mind that other engine problems, particularly overly tight valve clearances and ignition problems, can make you THINK that you have carburetor problems.  It is a really good Rule of Thumb to NEVER work on carbs, unless the ignition and valve clearances are checked first.

BMW/Bing finally gave up with modifications, and the later model carburetors worked fairly well. The final modifications and production changes on the R75/5 carburetors led to the first carburetors that GENERALLY worked fairly well, and were repeatable in operation, having model numbers 64/32/9 and 64/32/10. 

From that point on, carburetor improvements were in small changes from time to time. These /9 and /10 carburetors had the mixture adjustments sealed via an O-ring, a different idle speed nozzle entirely, the internal bypass bore was enlarged and its placement altered, the float needle spring was changed, the diaphragm limited in movement, the corner cavity in the bowl for starting was ventilated to the OUTside, the enrichener went from one stage to two stages to a modified three stages, ETC.  The slide itself was changed...and later models again used a slightly different slide.

NOTE: If you have these /3 and /4 carburetors, be advised that you MIGHT have any combination...or NONE... of any of these changes, whether or not the dome is marked with a "C". Carburetors sent for overhaul to Bing might even have stranger combinations. 

What to do:
FIRST, of course, is to determine if you really do have carburetor problems!   If you do, or wish to know more:
1. It is VERY important that the slide tube (the round tube at the top of the SLIDE) be polished to a fairly high degree, for approximately 1-1/2 inches down from the top. I BEGIN by using about a 600 grit Silicon Carbide paper, with kerosene, finishing with finer and finer grades. If the tube is not too bad, then I start with about a 1000 grit. 1200 grit is barely OK, 1500 or 1600 is better, ending with crocus cloth is NOT going overboard here. The dome bushing area where this tube moves up and down in, must also be fairly smooth. BMW specifies a .07 mm (.003") clearance. I do not bother to measure this, but I do use a dowel, and, again, 600 or 1000 and up silicon carbide paper, to polish the dome recess that the tube rides into. Take your sweet time, do NOT do a quickie job here. Install the slide and recess clean and dry. Just because you think the slide and dome move smoothly together, does NOT mean that they do this when vacuum is, DO that polishing.  Be sure the dome fits squarely to the body. 

NOTE!!   In some instances of excessive wear, maybe not in some others!!, it will be very helpful to ADD the large soft coil spring above the diaphragm, as used on later model carburetors.   This is because in some instances (you can SEE this happening by looking into the carb from its rear facing air intake side, sometimes.  At other times the engine must be running.) the slide simply sticks too much and won't go fully to the bottom idle stop position.    Fix that by installing springs 13-11-1-335-324 above the slides.
Here is MORE about the springs(s), and this was for much later carbs, but really does apply here:

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.  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).
I have my own input on this.   Some folks prefer the more abrupt throttle action of the stiffer spring, which is about 115 mm long and has about 30 coils.  The softer springs are about 120 mm long and have about 20 coils.  Certainly for the R75 early carbs, having a spring there when idle and off-idle is having stability problems, is a good thing.

2. It is important that the slide body also be fairly smooth, and that the edges of the slide not be too sharp, rather that they are smooth. I use 600 grit for that, finish with 1200. I do this as the final item before cleaning the slide assembly and reinstalling it.

3. Set the needle to the 3rd notch from the top. If you have set the needle correctly, there will be a needle exposure of about 1.556" from the bottom of the slide, center area, to the needle tip. This is a great way to KNOW you are on the 3rd. notch. In fact, no matter what carburetor, I recommend that measurement, and that you write it down on your bike's specification sheet (You DO have one?).

4. The chokes (enrichener) on these have a single stage, so when synchronizing the enrichener cables, when choke lever is off, ensure that the carburetor levers are on the stops, and that a very small choke lever movement will start both carburetor levers moving at the same time. Ideally the choke lever will put the enrichener levers at the off stops when the choke lever is off, and at full on stops when the choke lever is at maximum. You may find that you cannot get this as close as you might like. Modifications could be done, but are not recommended unless you have insufficient cold weather choke available, hardly a problem with these large enrichener passageway carburetors.  Allowing a minimum of cable free play, perhaps 1 mm. 

5. The latest procedure for setting float levels, on all Bing CV carburetors, is to raise the float with a finger, very gently, and adjust the float so that the gas just stops flowing when the top of the floats are parallel with the bottom of the carburetor body. Due to the sprung type of needle, it is likely better to raise the float until the gas stops, then very slowly lower the float until gas flow JUST begins, and use THAT point as the parallel check.  For these carburetors, what is not widely known is that if you have the bike on the center stand, bowls in place with a good gasket, and fill the bowls from the petcock, shut off the gas, and then QUICKLY remove the bowl... you can also measure the fuel depth in the float bowl, and the optimum from the middle of the bowl, lowest point, to the fuel level, is 18-19 mm. This is for the STOCK one piece floats!  This should tend to agree with your finger method. Float level affects richeness/leanness. I have used 20-21 mm sometimes.   These values of 18-21 are NOT for much later carbs!

6. If you are using the "shorting of spark plugs method" in adjusting the carburetors, THAT is NOT done when adjusting the idle MIXTURE!!  Clymers, etc. is WRONG!!! NO adjustments can be made, other than initial crude eye-ball mechanical ones, until the bike is RIDDEN at least 10 miles!!! That is true for all airheads, but ESPECIALLY critical on these /3 and /4 model carbs....and even somewhat the /9 and /10. 

7. The idle mixture must be readjusted any time the idle STOP screws adjustments are changed. The idle mixtures and idle stop screws interact with each other, less and less so as you approach the ideal adjustment for all. This is TRICKY on these old carburetors. 

8. Due to variances in engines and carburetors, sometimes these carburetors will adjust best with an engine idle speed of about 800. Some may require closer to 1000. Generally, if adjusting at LESS THAN several thousand feet of altitude, set the idle speed around 900. This is quite contrary to my recommendations on the much later carburetors and engines, where about 1050 is about right. Again, adjustments are far more critical on the /3 and /4 carburetors for idle, and there is NO way I know of to fix that problem. You may have to experiment with the idle speed. Generally if the engine is not over 1300 at sea level and not under 850 at perhaps 6000 feet, that is good enough! 

9. If you happen to be overhauling the carburetors, lap the top of the carburetor body itself on a surface plate, piece of glass, etc., with upside-down 220 grit sandpaper, figure eights, equal pressure, moving all around, to flatten the top of the carburetor body. You can finish with a finer grit. This will reduce any gasoline stains and seal the diaphragm better. JUST enough to clean the surface nicely. 

One further modification is possible. I suggest not doing this unless you have done everything else first, know that your
valves are good, and set right, timing correct, and that the problem IS in the carburetors, with idle irregularities
(and, you HAVE installed the coiled springs): 
1. Remove the slide. If the diaphragm and needle are still attached, be especially careful when doing this
    modification.  Under NO circumstances must the diaphragm be injured, nor the needle bent.
2. Obtain two common ROUND headed [not tapered under the head] screws, 4-40 thread, 1/4" long
    (under head to end). Steel is best, although brass or plated brass is usable. The head on the screw
    should be about .078" thick...although this is not critical, nor is the width of the head. 
3. Look at the bottom of the slide. You will see two equal raised areas, that the slide bottoms on, when the
    slide is fully down. Very carefully, with a sharp center-punch, put a punch prick at the center, radially
    and across, ONE of those flat areas. NOT injure the diaphragm, needle, nor nick the slide,
    ETC.  Drill, straight and squarely, a hole at that punch prick, using a #43 drill, depth to roughly 1/2". 
4. Drill the same hole, but use a #31 drill, and drill to ONLY about 1/16" inch in depth.
5. TAP the hole using a 4-40 tap. Be sure to do this straight and squarely.
6. Clean out the swarf, and install the screw without the Loctite, be sure the screw WILL screw down fully.
    Use a tiny drop of Loctite Red (#271) on the screw threads and install. Tighten the screw flush with the
    slide bottom. Be SURE it is flush, and both carbs identical.
7. Reassemble the carbs, resynchronize the choke cables, synchronize the throttle cables as best you can
    by eyeball for equal pull off the idle stops, leaving some slack, roughly equal in each outer sheath.
8. Go for a 10 mile minimum ride. Recheck the idle mixture and idle speed, adjust as required, then repeat
    the cable adjustment by your normal method.
9. If your cables are quite worn, you will NEVER get a good, repeatable, synchronization.   Earliest cables
    were not lined.

1.  If you have done most everything, and you have problems starting the bike (especially when cold),
     consistently hard starting, or maybe ONE carburetor is acting up, that cylinder not firing, until throttle is
     opened.....check to see if the butterflies were properly installed.  Not only is this to include loosening
     the screw whose bracket keeps the shaft in place, but loosening the butterfly screws (Usually not a
     problem even if peened, as you can usually loosen them a tiny amount; if not, grind, and replace, or
     when tightening, use Loctite BLUE), and be sure the butterfly will completely seal to the carb
     bottom.....and tighten things again.   If the butterflies are reversed, you will never get proper


2.  If you have a R75/5 that is particularly a bear to start when COLD, and everything else checks out fine,
     be SURE to check the slides, to be SURE they are BOTTOMING fully, and not hung up slightly, as it
     may be that the slides are not returning fully.  Fix that by installing springs 13-11-1-335-324 above the
   That number is for the 40 mm carbs.  If you have 32 mm carbs (obviously not on a R75/5), and
     think you have a slide problem, and there is no spring above the slide, you can polish the slide and carb
     interior area, and try spring 13-11-1-338-134.

3. Choke cables:  The earliest /5 carburetors not only had the return springs wrapped AROUND the
    throttle cables at the carburetors, but they had a different setup on the choke (enrichener) cables. 
    Earliest carburetors (before /9 and /10) had stranded-wire choke cables, and an INternal return spring.
    Operation could be iffy.   Cables 12-23-1-252-826.  The /9 and /10 carbs changed to solid cables.  You
    can convert, using 13-11-9-057-338 clamp screws.
        A full conversion could also consist of using (for both carbs):
        2 each clamping screws 13-11-9-057-338
        2 each lower fitting 13-23-1-256-295
        2 each upper fitting 13-23-1-256-019
        4 each nuts 07-11-9-057-338



Bottom Line:   If you do EVERYthing in this article...slide mod, spring, ETC!...and you still have problems that are serious enough to make you want to get later carbs, then, do so, and plant flowers or make an art object with the old ones.  HOWEVER, in a high percentage of instances, I have been able to get the /3 and /4 working fine (sometimes some things apply to the /9 and /10 too!).  BE PATIENT!
(NOTE!....I have had owners say they have done everything and results are not improved, so they are going to get new or good used later model carburetors.  I have sometimes, rather often actually, found out that these folks DID NOT check valve clearances, intake hoses, ignition timing, I HIGHLY RECOMMENDED THEY DO FIRST!)

04-09-2003:   Considerably edited for clarity, including much moving around of sentences and paragraphs.
                     Add .htm identification.
10-05-2004:   add ADDENDUM, and item 1 of same.
03/27/2005:   Addendum #2
05/14/2007:   Minor editing, split into 2 different sections, add part numbers where applicable
05/24/2011:   edit for clarity
05/14/2012:   add butterfly photos
05/17/2012:   expand information on the top of slide springs.
09/26/2012:   Add QR code; add language button; update Google ad-sense code; add missing Google
                     analytics code.    Go over article, clarify a few details, add a link, etc.
10/03/2013:   Add Special note near top regarding ignition unit (ATU).


Copyright, 2013, R. Fleischer

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