The ads above are Google-sponsored;
clicking on them at every visit helps support this website!
Clicking on something inside an advertisement helps even more!!


.... problems, including idle problems.

Copyright, 2013, R. Fleischer

A basic MINI-overhaul can be done with the carburetors in place on the engine.  This is often quite enough.  Three articles, the one you are reading, AND.... ....and (which is article 3B) are not just about how to overhaul a Bing CV carburetor.  These articles are specifically designed to fully inform you what you need to know, as an Airhead owner/rider, about these carburetors.  


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 & /10 carburetors are different
from these /1, /2,  /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, ETC!

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, & the ignition timing be correct.  Be sure there are no vacuum
leaks at the throttle shafts, hoses between carburetor & cylinder head are
tight, & NOT leaking vacuum (spray with any volatile solvent...must NOT
have idle rpm change!).
NEVER work on carburetors unless you are SURE all else is OK!!!

Set idle rpm to 1000 when adjusting the carburetors.   Maximum advance to
the F dot on the flywheel occurs ~2000 rpm on the earliest ATU.  The VERY
SLOW idle timing mark, that is, the "S" mark, should appear below 800 RPM
on an EARLY /5....that means, on the EARLY type 2000 RPM maximum advance
type of ATU.  You must slow the engine to BELOW 800 rpm during checking
of the slow idle timing (which is not critical, just in the window is OK).  
Unless you plan to run 91 or higher octane fuel, I recommend you change
the ATU springs to the later very slightly stiffer type, that give ~3,000 RPM for
maximum advance.   Re-said:  If the Automatic Timing Unit, ATU, has the 
early softer 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 2000 RPM advance version. 
However, that version allows some small ignition advance from ~800 rpm. 
Since, in MY opinion, the idle rpm should be within the range of ~900-1050, in
order to get the early carburetors to work properly (and get proper oiling to
the timing chest items inside the engine) that means some advance will be
had even at 1000 RPM idle.  You can have a buddy use the timing light and
you slow the engine below 800 RPM by dragging the clutch, transmission
in some gear, brake on hard, while he checks the timing marks. That
complicates things, & is further complicated if the advance springs have
stretched-out some. If stretched, the ATU will be unstable at low rpm,
because that allows SLOP in the advance weights (easily seen/felt,
engine OFF), that is, the springs are too stretched to allow the weights to
fully return at engine off.    The difference in acceleration/performance is
small between the "2000" rpm springs and the "3000" rpm springs,....
I think
it BEST to use the 3000 springs, no matter the gasoline, ...unless you are
doing serious modifications of the engine for higher performance, such as
a change in camshaft and/or lightening the flywheel, ETC.

Models with the 3000 RPM springs do NOT begin to advance at 800
RPM, as did the original early /5 ATU's.  The 3000 RPM springs make the
ATU start to advance around 1500 +- RPM, and finish advancing around
3,000 RPM.


This article deals with problems on the early versions of the CV carburetors
used on the R75/5; getting the bike to start and to idle correctly, ETC., as
best possible.

As many of you know, if you have followed the Airheads LIST for a long time, I wrenched
during 2001 on an early (12/1970) SWB R75/5 with the original carburetors, that had
NOT been so nicely overhauled in the recent past by Bing themselves
. Putting this
mildly, Bing miss-drilled two holes, & generally made a mess. Even when originally
delivered brand-new, these early carburetors had some badly made (& designed) parts,
besides the problems with them being an early design that was NOT well sorted-out.  
This was not hardly 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 early seventies when the /5 was still being
sold new by dealers; and I was the Chief Tech (and head of the shop) at one of those
Dealerships.   But, this particular set of carbs, in 2001, was a real mess. I ended up
trying just about everything; including all I had learned 30 years previously....and
MORE!  Because this bike belonged to a friend, I spent a LOT of time on the bike,
doing many things to it....especially working on those carburetors!   I used it as a test-
bed, with owner's permission, to try anything I wanted, to fix those carburetors.  This
set of carbs was the early type, with poor enrichener design, leaky dome, bad slides,
poorly designed idle passageways (at butterfly closing position)...and MORE!

One of the annoying things about this /5 that I strove to fix was a goodly tendency
for wandering idle & also sometimes having to use the choke to start the engine
when the engine was already hot from riding (and sometimes these ornery carburetors
would start when COLD withOUT using the enricheners!). Trying to get a solid &
stable idling adjustment on these very early CV carburetors is OFTEN difficult. Most
folks give up, install later carburetors, perhaps from a R75/7, not a bad idea actually,
if obtained reasonably priced. Almost any of the later Bing 32 mm carburetors will fit
and can be jetted to work OK on the /5 bikes.  While the owner was willing to purchase
/7 carburetors, I wanted to keep the bike as stock as I decided to see
how frustrated I could get!   I am ornery, dogged, & nerdy [among my better attributes],
& 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 somewhat poorly-designed simple single stage
type, 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.  Wrongly
installed butterflies will NEVER allow proper adjustment and operation!

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).
UNfortunately, Bing's drilling, because THEY had overhauled these carburetors, was

Some materials in these carburetors were absolutely not conducive to smooth &
consistent operation...the slide guide pilot was steel, later models are brass & hard
chrome plated. The choke (enrichener) was a sort-of one-stage type, poorly sealed
as it had no gasket.
  Carefully re-working the sealing surfaces worked fine, but I liked
to install the later gasket anyway.

Originally the 2nd needle position was used, & 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; I won't get into the /1 and /2, as you are very
unlikely to see them.


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, & 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 & 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, & 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
& now had 3 different sizes of fuel feed bores. 

It was in November of 1971 that BMW first ...and almost lastly....recommended 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), & 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 & /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,
& an irregular idle.
In some cases, the idle STOP screw setting will allow the butterfly to close
quite far, & 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, & 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 & stable operation. Even then, they are not as nice as
the later models, & 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 & new float, float needle, slide needle, slide needle jet,
diaphragm, & probably shaft O-ring & 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); & know how to adjust carburetors.  If not, read my other carburetor articles!

BMW made a number of modifications to these carburetors themselves; or, dealers did, &
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, & the enricheners, & 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. 

If you cannot get your /3 & /4 carburetors to work right after some decent attempts, perhaps
you should plant flowers in them.  HOWEVER, MOST THAT HAVE FOLLOWED MY

BUT, do keep in mind that other engine problems, particularly overly tight valve clearances &
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 & valve
clearances are checked first.

BMW/Bing finally gave up with modifications to the /3 and /4, & the later model carburetors
worked fairly well; they incorporated some of the things Bing and BMW recommended for
the original carburetors, plus passageways changes, & much more. The final modifications
& production changes on the R75/5 carburetors led to the first carburetors that
GENERALLY worked fairly well & were repeatable in operation, having model numbers
64/32/9 & 64/32/10.  From that point on, carburetor improvements were small changes
from time to time. The /9 & /10 carburetors had the mixture adjustments sealed via
an O-ring, a different idle speed nozzle entirely, the internal bypass bore was
enlarged & 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.

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. 


FIRST, of course, is to determine if you really do have carburetor problems!  
Assuming you DO have carburetor problems, not fixed with fresh diaphragms, fresh
floats, a close look-see a the butterfly fit, and you have looked into the enrichener, etc.....

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, note 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 & recess clean & dry. Just
    because you think the slide & 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. 

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:

    Tom said that this spring modification, installing a longer & 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 & has about 30 coils.  The
    softer springs are about 120 mm long & 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, & 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. Write it down on your bike's
    specification or service records 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, & 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
    mechanical stops when the choke lever is off, & 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, & 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; 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, & fill the
    bowls from the petcock, shut off the fuel at the petcocks, & then QUICKLY remove
    the bowl... you can measure the fuel depth in the float bowl, & the optimum from the
    middle depression area of the bowl, 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 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 & /4 model carbs....& even somewhat the /9 & /10. 

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

8. 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 & seal the
    diaphragm better. JUST enough to clean the full surface nicely. 

9.  One further modification is possible. I suggest not doing this unless you have
      done everything else first, know that your valves adjustments are correct,  ignition
      timing correct, & that the problem IS in the carburetors, with idle irregularities ..&
      you HAVE installed the coiled springs: 
     a.  Remove the slide. If the diaphragm & needle are still attached, be especially
          careful when doing this modification.  Under NO circumstances must the
          diaphragm be injured, nor the needle bent.
     b.  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. 
     c.  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 & across, ONE of those flat areas.  Do
          NOT injure the diaphragm, needle, nor nick the slide, ETC.  Drill, straight & squarely,
          a hole at that punch prick, using a #43 drill, depth to roughly 1/2". 
     d.  Drill the same hole, but use a #31 drill; drill to ONLY about 1/16" inch in depth.
     e. TAP the hole using a 4-40 tap. Be sure to do this straight & squarely.
     f.  Clean out the swarf, & install the screw without Loctite, be sure the screw WILL
          screw down fully.  Remove the screw, and use a tiny drop of Loctite Red (#271)
          on the screw threads & re-install. Tighten the screw flush with the slide bottom.
          Be SURE it is flush, & both carbs identical.
     g.  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.
     h.  Go for a 10 mile minimum ride. Recheck the idle mixture & idle speed, adjust as
          required, then repeat the cable adjustment by your normal method.
     i.   If your cables are quite worn, you will NEVER get a good, repeatable, synchronization.  
          Earliest cables were not lined.

1.  DO CHECK THIS!  If 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.....and particularly if you have irregular idle:  Check to
     see if the butterflies were properly installed.  BE SURE they are installed correction.
     IF NOT:   
         Loosen the screw whose bracket keeps the shaft in place.   Loosen the butterfly
         screws SLIGHTLY (Usually not a problem even if peened, as you can usually
         loosen them a tiny amount;.... if not,... grind, replace, peen, or, when tightening,
         use Loctite BLUE), and be sure the butterfly will completely seal to the carb
         bottom.....and before final tightening.   If the butterflies are reversed, you
         will NEVER get proper operation. 


2.  If you have a R75/5 that is particularly a bear to start when COLD, & everything
     else checks out fine, INCLUDING that the tiny jet in the corner of the float bowl
     is not SURE to check the slides, to be SURE they are
     BOTTOMING fully, & 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 slides.  
     That number is for the 40 mm carbs.  If you have 32 mm carbs (obviously not on
     a R75/5), & think you have a slide problem, & there is no spring above the slide,
     you can polish the slide & carb interior area, & try spring 13-11-1-338-134.
     Information on the springs is much earlier in this article: LINK  

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 & /10) had
     stranded-wire choke cables, & an INternal return spring.  Operation could be iffy.  
     Cables 12-23-1-252-826.  The /9 & /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


    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!
    I have had owners say they have done everything & 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 do everything, and may
    not even have checked valve clearances, intake hoses, ignition timing, I

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).
02/12/2016:   Justify left. Increase font size.  Clarify details.  Update meta-codes.  Clean up article entirely.


Copyright, 2013, R. Fleischer

Return to Technical Articles LIST Page

Return to HomePage

Last edit of THIS page: Sunday, May 08, 2016