EARLY R75/5 BING CV CARBURETORS
©
earlybingR75CV.htm-6
***The information in this article should be read by anyone that works on later model CV carbs too...as some things are good to know....such as about the springs that might be added, and many other things.
INTRODUCTION:
This article is
in TWO sections. This first section deals with problems on adjusting the
early version of the CV carburetors used on the R75/5; getting them to idle
correctly, ETC., as best possible. The second part of this article,
located WAY down this page, deals with other things dealing with these early
carburetors.
As many of you are aware, 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, these early carburetors had badly made 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 /7, not a bad idea actually, if obtained reasonably priced. I had not played with this series of
carburetor for a long time, and had the time to play a lot, and while the owner
was willing to purchase the /7 carburetors, I wanted to keep the bike as stock
as possible...so 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 mostly accomplished, with
considerable effort.
These early model Bing 63/32/3 and 64/32/4 carburetors were a bit crude. The idle passageways in the carburetor are a lousy design simple single stage type and are somewhat abrupt in operation as the throttle is just barely opened from idle stops. That is the primary reason that the butterfly's had better be installed correctly. Even on much later models, the positioning of the butterfly is very important.
On these early /5 CV carbs, Bing put some idle holes in a slightly wrong place.
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 long article.
O-ring sealing was not as extensive.
The original early diaphragms were thinner, replacements are thicker. There
are 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 as noted 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 troublesome carburetors is 64/32/3 and
64/32/4.
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 too...as some things
are good to know....such as about the springs that might be added, and
many other things.
Certain 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 reason...at 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 airtubes; keeping cylinder timing variation to under 1
degree (yeh, RIIIGHT....this also meant playing with the cam nose end....VERY dangerous except for experienced wrenches...although BMW
said to STONE the automatic advance cam so timing did not vary between cylinders
by more than 1°...laughable, as cam tip runout 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 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, even a changed float needle spring.
As just an idea to you folks about some changes that are esoteric, besides very tiny passageway size changes...well...how about that
float needle spring change....and a change in the ventilation of the ''starter cup'' from outside to inside?...or, mixture bore in the
rotary valve?? Anyone remember this stuff besides me??
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 carbs 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.
One thing I will mention only once here: Be SURE that if the
butterfly's were removed, no matter by who, that
includes BING!, that they are installed
correctly!
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.
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. 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.
BACKGROUND:
NOTE: 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 the 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).
NOTE that Bing's 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.
BMW/Bing finally gave up with modifications, and the later carburetors worked fairly well. The final modifications and
production changes 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 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 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:
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 present...so, 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 much
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) 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.
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. Yes, 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
800 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.
AGAIN...do 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, using a tiny drop of Loctite Red (#271) on the screw threads first. Tighten the screw
flush with the slide bottom. Be SURE it is flush, and both carbs identical.
You may want to try the screw without the Loctite first, be sure the screw WILL
screw down fully.
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.
ADDENDUM:
1. If you have done most everything, and you have problems starting the
bike, 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 (a problem if peened, if not, 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 operation.
2. If you have a R75/5 that is particularly a bear to start,
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 slides.
That number is for the 40 mm carbs. If you are reading this and have 32 mm
carbs, 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.
This is section two of this page:
1. 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
2.
Revisions:
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