Bing CV Carburetors, Part 1 of 2 parts
© Copyright, 2014, R. Fleischer
For a well-done overhaul article...step by step...with 44 pictures....see BMWMOA magazine, BMW-ON (BMW
Owners News), March 2003, for an article in great depth by Gary L. Smith. You can probably get that back issue at http://www.bmwmoa.org. From that main page, click on left side for Country Store, and then on the Country Store page, go to Back Issues.
I have SOME nitpicking on that article:
1. See my notes in the article you are reading.
2. Use faint amount of silicone grease on all O-rings & on enrichener parts (do NOT overdo this, you do NOT want to plug jet holes).
3. If doing a complete overhaul, which involves removing the butterfly to replace the throttle shaft O-ring (be sure to silicone grease that one & be SURE to MARK the butterfly for correct refitting: something like TOP, OUTER; or TOP, facing REAR...or something of that sort....BEFORE you remove the old one. You can hope that the previous workman did not install yours backwards, because it can be fun to figure out unless you know what it is supposed to look like. I ADDED, in the article you are now reading, on 05/14/2012, photos of what it IS supposed to look like..... & NOT look like.
4. The article you are reading, below, has a LOT more complete information about orienting the enrichener parts.
A basic MINI-overhaul can be done with the carburetors in place on the engine.
This is often quite enough. Two articles, the one you are reading....and http://bmwmotorcycletech.info/bingcv-2.htm (which is article 3B) are not just about how to overhaul
a Bing CV carburetor. These two articles are specifically designed to fully inform you what you need to know, as an Airhead owner/rider, about these carburetors. The one exception is if you have an R75/5, which has a particular, and troublesome version of the Bing CV carburetor. For those carburetors, you should review both of these above two linked articles AND, in particular, THEN review this one, which is article #6: http://bmwmotorcycletech.info/earlybingR75CV.htm
What follows is NOT a step-by-step how-to article. This article DOES cover every important thing you really need to know about working on Bing CV Carburetors....and, used together with the above two articles (links, above), you will be quite well-informed.
NOTE!....there is no overhaul nor adjustments article on this website for the Bing model 53 'slide' carburetors, as used on the R50/5, R60/5, R60/6, and R60/7. They are straightforward. They use the SAME floats as the CV carburetors. For acceleration pump parts check with both BMW & with Bing Agency. Some Bing carburetors, such as the R60/6 and R60/7, use an acceleration jet assembly in the
central assembly, these parts all come out mostly at one time, same as those carburetors without. One of the Airlist Members made some 'washers' that fit the acceleration pump, making them of nylon material. 3.556 mm x 0.33 mm.
I have not confirmed this works/fits.
READ Article #6 on the R75/5 carbs....some really IS pertinent to all the CV carbs:
That is Part 2 of the article you are reading now.
That is all about the STOCK floats.
That is all about Bing's "Alcohol-proof" float Kits.
Website in which you can enter your carburetor model number, and get a chart showing all the component
The Bing CV carburetors are complicated in HOW MANY PARTS OF THEM WORK. BUT: There are a limited number of places in these carburetors where you you might run into trouble. For the 'Newbie', I advise reading all of the carburetor articles completely through, even though you will not likely understand everything at first read. After having overhauled your first Bing CV carburetor, you will wonder what all the fuss was about.
A few notes to begin with:
The float needle tips tend to get faint grooves in them after a lot of miles. If a rubber-tipped type
the tip "rubber" material gets slightly harder, & gasoline tends to slowly (modern gasohol especially)
deteriorate them; then the needle tip does not seal well, causing the bowl to overflow onto your foot.
This can also happen if a TEENY MICROSCOPIC particle of dirt gets into that float tip & mating seat area; and if the float deteriorates enough. The stock float & float needle probably should
be replaced every 30,000 miles. It is VERY common to see a carburetor overflowing onto your boot.
Most Airhead owners have had the problem at one time or another. The fast fix is to turn off the fuel
petcock(s), remove a bowl & jiggle the float up & down with the fuel ON, to flush out any particulate
matter in the float needle-seat area. It is important to keep the fuel tank clean, & as the tank ages,
the interior reddish-brown lining deteriorates, clogs screens, etc. The rubber fuel hoses also
deteriorate INternally, allowing teensy bits of rubber material into the carburetors, where they can
cause float needle hangups, as well as plugging the idle or main jet. Use of aftermarket fuel filter(s)
below the petcocks is almost a necessity now, with the aging bikes. Information on screens, filters,
updates, etc., is elsewhere's on this website.
NOTE: Replacement of a float needle SEAT is rarely needed. Detailed information is here: http://bmwmotorcycletech.info/bingcv-2.htm
Your stock floats are held into the carburetor by a pin that is knurled on one end. Pins can wear, although it takes a VERY LONG time. If yours are worn, replace them. Be sure to push the pin out in the proper direction!! A magnifying glass may be needed to see the knurled end when still assembled. The pin MUST be pushed OUT with a tiny drift or even a modified nail AT the NON-knurled end. When reinstalling that pin, this is not the time to hamfistedly use ViceGrips or Channel-Locks, YOU WILL break the float pin bosses. You install the NON-knurled end first. Except for quite early models, all later floats are tied to the float needle with the earlier mentioned, & easy to loose, very tiny wire clip. That clip ensures the float needle releases from the seat, & some other reasons, already discussed. It is a PIA to install the stock floats...or the KIT bridge...with that wee tiny wire clip & the float needle. Just takes practice....see below...
That clip is not used on the models 64/32/1-20; but it is possible to install them though.
Re-hashing things: The later float needles lower end has a spring inside & a tiny plunger. The plunger has a tiny hole in it, that the wire clip fastens to this end part, & the plunger rotates easily.... some may find it a more than a little bit of an annoyance reassembling these. When installing the float, float pin, float needle & that tiny wire clip, they must be installed as an assembly, a bit tricky, especially if the carburetor is right side up, but doable. I suggest a white sheet under the carbs, in case you 'loose' that wire clip or needle. I highly recommend that you order SEVERAL of these wire clips when you order floats & float needles, as they are SO easy to lose. ALWAYS replace the float needles when replacing the stock float & vice-versa. When the float needle tip gets bad, the carburetor will leak on your foot, the mixture gets very rich, & the fuel mileage goes way down.
***A very rare problem is a float needle that seats irregularly, yet a new needle, sometimes someone
even replaces the needle seat, does not fix the leaking. INSPECT the float tang...it needs to be flat
and NO irregularities in its approximate center where it contact the bottom of the float needle. This
is very rare since most people have to replace floats long before this wear is noticeable.
There are a number of rubber O-rings used in the Bing CV carburetors. All should come with the rebuild kit (5 O-rings). These are BMW part numbers and sizes:
13-11-1-257-739 Used at the throttle shaft, 6 mm x 2 mm and rather thickish.
13-11-1-257-819 Used at the enrichener shaft, 5 x 1.5 mm and rather thinnish.
13-11-1-257-812 Used at the main jet assembly, 8 x 1.5 mm. (measure: ~1.75 mm)
13-11-1-254-735 Used at both the idle adjustment screw and on the idle jet, 3 x 1.75 mm
The screws for the carburetor top cover are M5 x 12 oval head. They are usually EITHER Phillips head...or a similar looking but differently shaped cross type of screw. I get into this later.
Except for the /5 models (unless modified for this), lighter carburetor springs are available....they are not the same for 32 & 40 mm carburetors. Check with your dealership; or, Ted Porter's Beemershop in California. There is a type 606 for the 32 mm carburetors that have 3 digit model numbers; a type 908 for the earlier 2 digit 32 mm types, and a type 312 for the 40 mm carbs. Be sure your cables & carburetor levers do not have excessive friction, or these lighter springs will not work properly, that is, the levers will not return all the way, if throttle or enrichener (choke) is moved OFF.
There have been some problems with the R45, R65, & POSSIBLY some of the larger engines, from 1979 or so, for a year or three after that time. The information from the factory on this problem is sketchy. The problem is poor throttle response of various types, during initial starting & riding off, during quite cold temperatures. Basically, the enrichment device in the carburetors is not rich enough. BMW (Bing) changed the enrichener disc (that disc with the 'funny' holes) by, essentially, enlarging some, blocking off some or adding some. NO change in part number was used. I have the bulletin on it, which, in essence, says to use a new disc, or, modify your disc, by drilling it with specific sized drills, and exactly which/where. I have not posted the SI (are two) on this website, because there are other more likely possibilities for cold starting/running (until the engine has been running a few minutes), such as clogged bowl jet, etc....and, the fact that almost no one rides in such extreme weather, & is not likely to be bothered by a seemingly somewhat slower warm-up period, and, the danger of them drilling the wrong place, ETC.
Diaphragms, and rubber parts:
I recommend that the diaphragms be replaced at around 30,000 to 50,000 miles, possibly even
70,000. Many have gone MUCH longer, but I don't know of many in that category when using
gasoline containing ethanol.
Some of this variability has to do with type of fuel, or atmosphere (smog, etc.), and some with time, some with mileage. I have no objection if you let the diaphragms go until they fail by getting a hole or tear in them. You usually must remove the slide/diaphragm assembly; stretch the diaphragm a bit with your fingers while holding it up to a light to see holes & tears; but sometimes they are very obvious without the removal & stretching. You can purchase the individual parts if you need or want to. Diaphragms seem to hold up reasonably well over time, so I cannot give a replacement period just for time alone. The type of components in your gasoline seems to have some effect on their longevity. The very earliest /5 diaphragms were replaced by a thicker type, which are all that is available now. Some of those very early ones might still be around.
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!...except in emergency.
32 mm kits from BMW are 13-11-1-258-051
40 mm kits from BMW are 13-11-1-336-902
Do NOT!!!...use anything but BMW Dealership or Bing Agency 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....unless in emergency.
With some Airhead carburetors BMW does not offer just the diaphragm & 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. If you want to, you can purchase the diaphragm from BINGUSA. You CAN pry the ring off without breaking it ....if you USE VERY HOT WATER, EVEN BOILING WATER, to soften & expand the ring. Be gentle, do not try to open the ring too far.
I 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. It would
be better if you obtained the diaphragms from BMW dealerships too. BMW dealers
prices are generally CHEAPER than Bing prices.....and you can NOT depend on Bing USA
parts being of the same quality (in several ways) as the BMW sold parts.
Be careful! Do NOT use Stromberg diaphragms except in emergency. I have a few reports that seem to indicate that Bing themselves, or places that sold them saying they are Bing parts, might have substituted Stromberg diaphragms at times.
NO PROOF, yet.
Some BMW/Bing diaphragms are a bit thinner than many years ago. I am NOT talking here about the previously mentioned early /5 carburetor thinner diagphragm replaced by thicker. If the groove in the top of the carburetor body where the top casting meets the diaphragm & seals the diaphragm to the carb body is too deep, then there may be leaks. This has happened even with the correct thick diaphragms. The result may or may not be dried fuel stains on the top of the body, but may also, especially with the thinner diaphragms, be revealed as a LEAN RUNNING CONDITION, perhaps in the mid-range. You can, if experienced, 'feel' the leak by moving the slide up and down with your fingertip, carburetor fully assembled. Only a very experienced technician knows that THAT is SUPPOSED to feel like. If you have MIDRANGE lean mixture surging, fixed by moving the needle up one click, then: Remove the top and diaphragm/slide, and sand the carb body until the groove depth is ~0.146".
Screwdrivers....for Bing CV carbs:
(yes, a whole section on SCREWDRIVERS...FOR TWO SPECIFIC BING SCREWS! (well...maybe some additional information....)
WHY a section about screwdrivers, and Bing carbs?....well, I am trying to sneak
into this BING CV article information about how to remove the carburetor tops without
damaging the screws, and how future removals will be much easier and proper
use of proper tools SHOULD BE known, and practiced, by those who want to own
and ride one of these classic bikes.
The top screws are M5 x 0.8 x 12 oval head
BMW supplies a screwdriver 71-11-1-103-086 in the on-bike tool kit. "Phillips" & standard flat blade ends, reversible. The 'Phillips' end is NOT a PosiDrive nor is it a Reed & Prince tip. There are better choices, stronger, longer lasting, 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 and budget, the Snap-On is a QUALITY tool, that almost never wears out the tips. Be sure you have the proper size tips. The proper Phillips size is #2 for the carburetor top screws WHEN they are Phillips.
BMW carb top " phillips" screws are not always really Phillips screws, they just look that way at a quick first glance. They could be Phillips, OR could be 'PoziDriv' type'...I suppose they could even be other shapes. Someone might even have changed them to allen head types. The ancient aircraft tip screwdriver called Reed & Prince (Frearson) works relatively nicely on the PoziDriv screws....as, of course, does the real Pozi-Driv. The PoziDriv tip is EXCELLENT for REMOVING the Phillips type, if the Phillips is way tight. Generally install a real Phillips type with a Phillips screwdriver, but the other types of screwdrivers DO work better. If the screws are frozen, you can try a variety of ideas. The Phillips type of screw was DESIGNED to not slip as much as others of its time...but, also designed so the TOOL WILL SLIP after a certain torque is reached. This is why removing a stuck Phillips screw is so annoying...especially after some wear on the screw or screwdriver. Methods of adding friction for easier removal includes valve grinding compound for a better grip.
For egregious instances, I use a metal block underneath and an Impakt Driver with the PROPER TIP. Interchangeable-tip tools are available from a variety of sources, including Snap-On. The PosiDriv tip IS available. Some have installed Allen head screws at the carburetor tops. They are OK, but don't over-tighten, as many of these have a very small allen & can round-out more easily. Some early carburetors had common slot screws. BMW & Bing may be, and have, shipped EITHER PosiDrive or Phillips screws......be sure that your 'screwdriver' fits them. Remove the screws one at a time, coat the threads...and under head taper...with antiseize....and replace the screws without too much torque;.....you will appreciate these hints much later on. 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 NECESSARILY THE CORRECT TOOL for the carburetor tops....this tool is a common Phillips, and not a good one. The PosiDrive screws generally have some radiating lines to indicate they are not Phillips type. Note, again, that the Pozi type screwdriver will usually work well on Phillips screws.
Bottom line: Try to use a screwdriver that, upon close inspection, really fits the screw.
Here are photos of the Posi-Drive screw, and the screwdriver tip. Note the differences from a Phillips....the nearly flat bottom in the threads and the corresponding flat top of the screwdriver; note also the angles and the extra (lesser) 'splines' in-between the major splines. If the screw looks the same, and no tick marks, it may be a SupaDriv.
A bit more:
The Frearson (Reed & Prince) screws used a screwdriver with a sharp tip, and a wider angle, thus the screwdrivers could be used on a wide range of screw head sizes. These are a very old (now) design, not really used since the seventies. The typical use was marine hardware. The Phillips had a more rounded and tapered shape. There is another name, often used with or similar to the PoziDriv (AND, note that this screw is actually NOT POZIDRIVE..there is NO E on the end), and that is SupaDriv. They seem the same, but while their respective screwdrivers will work with each other, not quite perfectly. The SupaDrive allows a bit higher torque than the Pozidriv, and the screwdriver can have a modest ANGLE to the screw....thus is nice for overhead use. The POZIDRIV has, tick marks, see photo above. USE OF A PHILLIPS SCREWDRIVER ON THESE SCREWS WILL DAMAGE THE SCREW.
Many diaphragms have a downward facing tab that fits into a small recess in the slide itself.
Diaphragms have a somewhat larger downward facing tab that fits in a corresponding slot
in the top of the main carburetor body. Tabs and slots must
line up during the actual fitting
of the parts, and it is easy to accidentally rotate the diaphragm when putting the carburetor
top back on. When assembling the diaphragms to the slides, be careful that you assemble
things concentrically and carefully. If the needles are still in place, be extra careful not to
Tighten the top of the carburetor screws evenly, coat the threads and underside with a WEE dab of antiseize, and do NOT overtighten.
With the central jets parts assembled, some care and wiggling may be necessary to install
the slide and needle assembly in order to get the needle into the lower brass tubular parts area.
NOTE that many have initially assembled the atomizer jet wrongly. In the Bing CV carb, that
stick UP INTO THE CARBURETOR THROAT. If you assemble wrongly and
then put a wrench and some force onto the central jet assembly, you can destroy your carburetor.
Here is more on that assembly:
The central jet assembly top-most piece tubular brass part is the atomizer. This loose part, as you begin reassembly, fits directly above the tubular part called the needle jet (which fits into it, and has such numbers on it as 2.64, 2.66, or 2.68; and works with the tapered slide needle, itself adjustable for height. This top-most brass part called the atomizer must stick up INTO the carburetor throat, and only ONE end of it has the correct diameter to allow it to fit up into the throat. A problem can come about if one has the slide, diaphragm, and its wiggly needle already in place in the carburetor, and you now try to install the brass parts, which is done from the bottom of the carburetor central area. Be sure that the black rubber O-ring on the central jet assembly is in good condition,... if questionable, replace it; using a wee bit of silicon grease to help its installation, and on the outside of the O-ring. Since the threads you must pass the O-ring over, could damage the rubber O-ring, I suggest wrapping some plumber's-type of thin Teflon tape over the the threads area, during assembly. DO NOT damage the O-ring with sharp tools.
Clean things, including the cavity, first. When installing the atomizer, ETC....be VERY careful that that the proper end fits into the throat, and that the needle does not catch the edge of the jet....you might not notice; then screw the jet assembly upwards, bending the needle...or...MUCH worse yet, ...applying too much force; breaking the threaded carburetor boss. NO excessive force is needed here!!! SO, if the slide with its needle is already in the carburetor, be especially careful installing the central jet assembly.
Slides are reinstalled into the carburetor clean and dry, and the lower jet assemblies that the slide needle fits down into, really should, ideally, ALREADY be in the carburetor!! If you are careful, see above paragraphs, you will be OK.
When assembled correctly, the slide, which has two holes at its bottom, off center, will have
those two holes facing the cylinder head. Slides work OK even when fairly well worn. Bing
has been offering some slides with O-rings. They are quieter, in a few instances of rattling noises. I am rather undecided as to whether I like them....or not.
Since wrongly assembling & using force with the slide, needle, and central jet, can damage the carburetor badly, I will get into this a second time, a bit differently:
When you install the main jet, and the parts associated directly above it, it is best NOT to install these parts after, BUT BEFORE you install the slide/needle/diaphragm assembly. Failure to follow this advice can lead to bending the slide needle, you can cause a real hang-up inside, which is hidden from view, and further tightening of the jet assembly using a 10mm wrench can cause you to, in the worst case, split the carburetor boss. This is nasty to fix, most folks just replace the carburetor, or, its body. Some do an epoxy job, which MAY or MAY NOT work. Sometimes a sleeve is made and installed, perhaps epoxied also. A new carburetor body is REALLY expensive ...unless you find a cheap wrecked bike to remove it from. Old hands at working with Bing CV carbs install any way they want to, as they know the feel, do it with fingers initially, and also have their other hand's finger moving the slide needle at the same time, typically lifting the slide fully up. This WILL work well...and is OK for you to do, just be gentle and watch what you are doing.
Rather often the brass atomizer part that sticks upwards into the carburetor throat does not fall
downwards and out when the central jet assembly is removed, or does it later when you are not
looking! Use a toothpick or similar to
dislodge it, or a tiny screwdriver carefully on the top
side, to push it downwards. It is easy to lose these parts, so do NOT!Remember, I recommended
an old piece of white sheet under the carburetor if on the bike. Once in awhile, that brass atomizer
part does not seem to want to go into position...it is usually just a wee bit of crud on it or in the
carburetor body hole. Insert the atomizer as squarely as you can after cleaning the hole and
atomizer, and it will install OK. Remember that ONE end is slightly smaller than the other, and
only that end CAN fit. As always, be sure all parts are clean when re-assembling the carburetor.
When one does a mini field-overhaul on a Bing CV carburetor, it is usually not necessary to totally disassemble the carburetor, removing every jet, every O-ring, the enrichener parts, etc. Normally, one really needs only to replace the diaphragm, float, and float needle and spray the passageways a few times with a good cleaner. If you removed the idle mixture screw, replace its O-ring, FAINTLY coating it with silicone greasefirst. Same for the pilot jet, central jet assembly; replace rubber O-rings. The main thing, though, is that the DECISION is: What needs replacing, what needs cleaning, and, do I really need to remove the carburetor butterfly and shaft. THAT LAST NEED is part of a THOROUGH overhaul...but, if there is no vacuum leaks at the throttle shaft (engine running,brake cleaner sprayed at the throttle exit from the carburetor...idle rpm change??)...and the throttle (spring removed) rotates nicely...you might decide to leave the shaft alone.
At some goodly mileage (Bing says 25K, I say ~60K), one should replace the slide needle and its companion, the needle jet. The reason to replace these is that the needle is designed to vibrate freely, and the two wear each other and change size, the result of which is a richer midrange. Yes, cheapskates can lower the slide needle one notch to sort-of compensate, but this is usually way too much. Most late model slide needles were aluminum...and the GROOVE wears very fast....they are to be replaced as soon as vertical looseness is noticeable. You can see the wear if the CLIP on the needle allows up and down movement. These types of needles are installed differently and adjusted differently, than earlier needles.
Early needles are SLIGHTLY pressured up or down with your fingers, as desired, and rotated 90°, to find the next 'position'. Later needles have the position set by a special type of C clip, and the needle is held-in-place by a large special screw, that screws down from inside the slide central tube (obviously accessed from the top of the slide). The later type of design is nicer, as you cannot goof on position, but the aluminum needles do not hold up like the steel ones. Wear occurs where they vibrate against the central jet assembly (the NEEDLE JET also wears)...& the clip area wears, and the needle becomes unstable...and then things begin to get quite bad as time goes on.
When installing O-rings, put one layer +- of some sort of tape, I prefer plumber's Teflon, over the sharp threads, & use silicon grease to ease the O-ring into its groove. Remove the tape afterwards. You can push the O-ring with SMOOTH fingernails on your thumbs, & if you have to, use a tiny SMOOTH dental type of metal hook to lift the O-ring into the proper groove position on the part; or, for the whole operation. This works really well with the idle pilot jet & idle mixture control & the central jet assembly. DO NOT NICK OR CUT THE O-RINGS! I like to smear a VERY FAINT amount of silicon grease onto the O-rings just before I install the part into the carburetor. Not only does the silicon grease (FAINTLY!...don't plug up holes, etc!!!) help ease the O-ring over the part...but I believe it lengthens the life of the rubber O-ring.
One should remove only the necessary parts, then spray into all the jets (pilot jet, mixture adjustment
hole, bowl jet, central main jet assembly), etc., with a strong
carburetor spray; let sit awhile, then
spray again in every direction possible through those holes. I prefer Berryman B-12, in a version
called 'Carburetor and Choke Cleaner' for this job. This is a very strong solvent mixture that actually
dissolves most all of the deposits from gasoline, which many other spray solvents do not. Common
brake cleaner spray is BAD, as it is not powerful, in fact it is quite mild for cleaning. You might
consider spraying all the metal pieces, then flush with a common spray brake cleaner or equivalent.
Flushing gets that strong stuff out of there. I do
recommend removing the central jet assembly, it
tends to get crudded up, often with 'black' stuff. Be careful, as has been cautioned above, when
replacing. There is an O-ring on it that must be in good condition. Same for idle mixture & idle pilot jet.
Removing the idle mixture screw and spraying all the idle passageways with that strong Berryman product is a good idea. Spray three times, waiting a minute or so each time. Use plain clear silicone grease, or Dielectric Grease, from your autoparts store... EXCEPTIONALLY LIGHTLY on anything brass that screws into anything (antiseize is also OK), and silicone grease for any rubber O-ring. Use of silicone grease will tend to protect the O-rings from being damaged when installed, as well as greatly lengthening their life, making things turn smoother. There are some VERY SMALL holes in certain passageways & jets; you do NOT want grease clogging them!
I use the silicone grease on the enrichener mechanical parts too, but be careful, you do NOT want the round disc jet holes clogged. It is helpful on the enrichened shaft, outer part of the disc, etc. FAINTLY on that disc; be careful about WHERE on the disc!
Some carburetors have the diaphragm held in differently. Be careful expanding any plastic rings/clips, they can crack. A small amount of heat from a hair dryer, on the plastic retainer, or in very hot water, is VERY helpful.
NEVER clean jets with tiny drills, etc. There is a danger here that you might increase its size. It is probably OK to clear a jet with a very thin wire, but be careful. Typically a wire is not needed, if spray solvent is left in place a few moments, and then re-sprayed.
"ENRICHENERS"...yes, a whole choking section on this subject:
This section may seem quite complicated at first, it is NOT, really. You simply need to be CAREFUL to assemble the carburetor in the correct positioning of the parts. >>Some carburetors were MISMARKED by the Bing factory<< ALL will be explained in this section. Read it through TWICE, and LOOK at YOUR carburetor parts.
A cleaning & VERY LIGHT silicone grease lubing will make various parts operation smoother. If the rotating disc, which has holes for jetting, gets plugged (unusual unless you use grease), the enrichener won't work correctly. A VERY VERY faint coating of silicon grease is appropriate on the disc; but a fair amount more on the shaft and hole the shaft goes through in the cover. Use the grease ONLY where parts are actually making contact. I use a Q-tip to apply it.
The enrichener pieces are easy to mix up & get installed backwards...or;
get the left & right carburetor enrichener parts mixed up. A good rule is to
do both left & right carburetors at the same time.
Your bike could
ALREADY have them wrongly assembled!...and they are occasionally
WRONGLY marked by the factory!!!!
The enricheners MUST be assembled correctly for the 'choke' to operate correctly, and to prevent other problems.
In the FLOAT BOWL, the corner jet in the 'well' must be clean & UNclogged....clogging is a COMMON problem.
There are two very tiny pipes that must be good, and UNSPLIT, one comes down from the carburetor body into the enrichener well of the bowl, the other tiny pipe is in the middle of the bowl and vents to the underside of the bowl. Once in a while water collects in the bowl, freezes, and these teeny pipes split. The enrichener pipe may or may not have a tiny round hole in its side.
Carburetor enrichener parts orientations:
The left side photo, first one below, shows a dot-dimple on the enrichener shaft, which, if correctly marked by the Bing factory and correctly installed, points to the cable-fastening end of the lever. Later model carburetors have two slightly different levers plates, with a notched slot for the cable, & hole for the cable barrel. I have a photos of them further down this article. It is possible that some shafts have the DIMPLE REVERSED by mistake, during assembly of the disc/shaft. This can happen for the right or the left carburetor. By using my photos, below, of the inside of the assembly, you can be sure your carburetors are correctly assembled, even if miss-marked or miss-assembled by the Bing factory....or, prior person.
NOTE!...it is also possible for the SHAFT to be wrongly inserted into the disc, some models. Please use all the photos in this article to compare with YOUR carburetor.
The disc on the shaft is spring loaded on those later models that, pressured lightly with your fingers, moves the disc in and out on the shaft a tiny bit. Remove the nut, which will allow the choke lever(s) parts to be removed, first noting how the lever parts were attached. If you assemble them wrongly, the cable barrel, etc., will not fit correctly!!! See a bit further down the page for a more detailed explanation and photos.
Below are two photos:
The one on RIGHT has shaft market R, and is from a 1978 Airhead RIGHT side carburetor. Early carburetors have the number of tiny disc jet holes and the elliptical-tail slot different, with some holes missing. There are at least two styles of those. Although it would be nice to have the dimple, see the left photo below, such that it really does point to the cable barrel as designed, it really makes no difference, so long as the cover with its parts fits the carburetor in the correct position. When I find a miss-marked shaft, I usually grind the dimple down & RE-MARK the shaft! The correct position, for the RIGHT-SIDE carburetor, is shown in the right photo of the two photos immediately below.
NOTE where the elliptical hole, and the small jet holes, align to the enrichener cover.
The lever shown in the left photo is the ONE-PIECE type used on the very early carburetors.
If you were to look at a LEFT carburetor disc and shaft (instead of the RIGHT, as below), you would see the tail of the elliptical hole pointing the other direction. Photos of the LEFT side are in this article also, further down after the second set of photos. NOTE...the right photo should NOT be construed to have its shaft/lever in the position of the left photo.
|Below photo is of the early type of lever.||RIGHT side, correctly marked & assembled|
|elliptical hole tail in correct direction. LEVER is barely noticeable next to the lower right hole edge, is pointing downwards.|
|Enrichener lever in the two extremes of positioning are in other photos, below.||
There are numerous types of these discs. SOME have a depression for
one or two of the 4 small holes shown in the right photo disc, yet they are not
drilled. The elliptical hole can vary slightly too in size. The discs are NEVER to be different between a left & right carburetor on the same bike.
The brass shafts operating the enrichener are stamped in the inner ends, L & R for Left &
Right carburetors. The stampings can be a bit vague, see above photo on the right. SOME
late models (well after early /5 CV carburetors) rotating thick metal discs have an elliptical
hole, & 4 smaller holes, one of those 4 MAY be a bit bigger, & some may have one or two
blind hole depressions not drilled at all. Thru holes MUST be clean/clear! There are numerous
types of the discs, some will not be machined with the elliptical hole through the disc...and
different holes sizes & arrangements & even a special large brass washer plate may be seen:
later versions may have a brass thin disc on the other side of the thick disc, it is not shown here.
Here are some photos that will further explain things. These are from RIGHT side carburetors:
If you were to have the LEFT carburetor enrichener unit off the carburetor, & put it in front of you, upside down...that is...you are facing the inner (disc) side...and oriented so the round protuberance of the outer casting is TO YOUR RIGHT, and the LEVER is UPwards to its stop...about 1:00; THEN, the elliptical hole of the disc is roughly opposite the upper left casting screw hole, say 11:00....and the 4 tiny disc holes are roughly to the lower right....say 5:00.
For the RIGHT carburetor enrichener unit, as in these various photos, for the SAME orientation of casting & lever...the disc is REVERSED...that is...the 4 holes are to the UPPER LEFT, & the elliptical hole is to the lower right.
The above photo, marked WRONG!! has the dimple in the wrong direction. This MAY be necessary if your DISC/shaft was wrongly assembled or wrongly stamped by the factory! PLEASE RE-READ THESE TWO SENTENCES AGAIN!!
Yes, you could press it all apart and fix the direction...but I suggest you do NOT. You could sand the dimple down, and remark the shaft.
Just below is a photo of a CORRECT LEFT side enrichener. This is from a rather late model, and you can see a tiny bit of the brass disc, through the left end of the elliptical hole. Note that the upper-most of the tiny holes is counterbored, but NOT DRILLED, thus, it is not operating as a jet. This varies with model.
There is an O-ring internally on later enrichener models. I lightly silicone-grease that O-ring.
Later model Bing CV carburetors have two-piece cable levers.
It is possible to WRONGLY
install the two metal pieces. It is annoyingly EASY to overlook this, and you won't notice until
you have the carburetor fastened to the engine. Lever parts can be installed upside down
AND/OR reversed in position. In BOTH Left and Right carburetors, the part that has NO notch
(no slot) for the installation of the cable wire, goes onto the carburetor enrichener or throttle shaft first, and note the enrichener here with its offset facing the carburetor body. The outer part, that
DOES have that notch (slot), can be installed wrongly, flipped-over if you will. Install it such that
the notch does NOT face upwards during cable operation. Here are photos of how the later
model levers and springs look when installed properly. Note how the short tang end of the
enrichener spring fastens to the top, and further notice how that spring is on top of the enrichener lever, not the underside. To see why I install them this way, actuate the choke and note that the spring is not interfered-with by the lever, etc., as it moves.
Controls side; RIGHT side late model
Controls side; LEFT side late model
NOTE that for a smooth AND PROPER enrichener operation, the enrichener needs to be faintly
lubricated, cables good, & operating lever cable barrel lubricated (IMPORTANT or cable can
break strands!); if you have them, the T barrels under the fuel-tank where the one cable from
bars splits into two, should be internally clean and in good condition.
The fuel bowl gasket MUST BE IN GOOD CONDITION for the PROPER operation of the enrichener. The reason for this is complex, but a simple explanation concerns the tiny diameter pipe that comes down from the enrichener circuit in the carburetor top body, and dips into the enrichener fuel well in the bowl. That tiny pipe MAY have one or two holes in it ABOVE the pipe lower open-end. There is also one or two holes where the pipe joins the carburetor body, not discussed here. Vacuum, during the starting sequence with enrichener ON (CHOKE as marked on the bars assembly) causes fuel to rise in the enrichener well in the bowl to a higher level than in the rest of the bowl, creating even more richness than might otherwise be possible. The operation of the holes, if any, in the tiny tube is complicated, so won't get into it. BUT: If the gasket is leaking air, the maximum enrichment could be LESS.
Due to the over-all enrichener design, the amount of enrichener to be used, & whether or not you need a bit of throttle opening during cranking/starting, and after just starting, can be rather variable. Most owners booklets say to not open the throttle, as it defeats the enrichener....A not necessarily true simplification. You very well MAY need some throttle opening immediately after the starter motor starts cranking the engine, and in the seconds during starting, and this depends a lot on the particular motorcycle tuning, carburetor butterfly adjustment; temperature, fuel volatility, etc. In cool or cold weather, full choke may be needed for starting AND, very contrary to all the 'books' nice verbiage, you MAY NEED to manipulate the throttle during cranking.
In one early version of Clymer's manual that I saw, in the early section on how
to start your motorcycle, Clymer's has the operation of the choke lever (on the early
models where said lever is on the clamshell of the air cleaner housing), BACKWARDS. The truth is that the lever must be HORIZONTAL for the choke to be OFF....and DOWN for
choke ON (starting). Clymers fixed this in later versions of this manual, although I did not
check all sections of the manual. Maybe I had something to do with fixing that first mention.
Very early Bing CV carburetors have pressed-in float bowl enrichener jets, not screwed-in.
The enrichener (choke) is held to the carburetor body by 4 screws. These screws are infamous
for loosening, and the gasket can then get sucked-in a bit, and the carburetor not work
correctly, even with the choke lever off. The problem CAN EXTEND to improper throttle
feel, idle irregularities (can be large ones), ETC. If the carburetor is still on the motorcycle, I
recommend, once the throttle & choke cables are removed from the top of the carburetors, that you
loosen the carburetor adaptor clamps & rotate the carburetor to allow the tightness of these 4
enrichener screws to be checked. If loose, either tighten; or, remove and use a wee drop of
Loctite blue on the threads; then reinstall & tighten them. They CAN be tightened without rotating
the carburetor, but it is typically a big hassle, even with several types of offset screwdrivers. If you
wish, you may use the faintest possible smear of some sort of NON-silicone gasket sealer on
both sides of the gasket. REALLY faint! Gasgacinch; Permatex Form-a-Gasket (non-hardening),
etc. NO RTV HERE. The reason is not to prevent leaks directly, but to help prevent the gasket
from sucking-inwards, which then DOES cause leaks. I do not use any such sealants, but I do,
during overhaul, make sure the cover... & body it fits to... are FLAT, with NO proud metal. I surface
the cover, disassembled of course, on 220 grit paper. I also use a very fine file, very carefully,
on the body mating area.
***It is OK to not remove the throttle butterfly valves unless the shafts are really worn or leaking, as they are O-ring sealed, the screws peened, & the shafts can still be leakproof with a fair amount of side play due to the presence of the O-ring. To test for leaking, spray the shaft area from the outside of the carburetor, where the throttle lever attaches, with brake cleaner, while engine is idling. The idle speed must not change. If you do change the shaft O-ring, DO coat it, a bit more heavily than faintly, with silicone grease. They will then last almost forever, & operate smoothly. If you DIP the carburetor into a cleaning tank for any period of time with the usual harsh carburetor chemicals as used by professionals, you will HAVE TO replace the shaft O-rings. If the shaft is removed, then you must decide about the screws, to replace or not, to peen... or.... Loctite them.
NOTE!!!>>>>>The butterfly can be put in backwards. Do NOT! That is why I
previously said to mark the butterfly!...top, rear...or similar. This is a CRITICAL point.
HOWEVER, I highly recommend you check yours,
as they could already be installed
The fitment of the lower edge (the sharp shape at that point) of the butterfly is critical
where it comes close to the the idle passageways holes at the carburetor bottom. Putting the
butterfly in backwards will eliminate any chance of proper off-idle throttle performance.
This is doubly important with a dual plugged machine in which the butterfly is even more closed at idle. If the butterfly is installed backwards you will NEVER have a good stable idle. When installing the
butterflies... as you slowly tighten the two screws, you must lightly press the butterfly with a finger,
into the main carburetor bore, to align the butterfly...it is important that it fits the bore very evenly,
and for minimal clearance (shine a light through the carburetor as you look from the other side).
You may have to unscrew the IDLE STOP screw at the throttle lever some, to close the butterfly
valve enough for a proper look-see.
If the butterflies are reversed, you will never get proper operation; especially you will have IDLE problems.
The later enrichener (choke) control and all throttle controls on all the carburetors, each have an outside return spring. The springs are not the same. The old style carburetors had the enrichener (choke) lever on the aircleaner housing, and springs were not needed as the cable inners were very stiff single strand of steel wire. The lever assembly at the air cleaner snail housing can be disassembled and cleaned and lubed, as they tend to get stiff with age. It may be necessary to adjust a size of a thin shim to allow smooth action...as you do not want the nut loose. Sometimes that area takes some fiddling with.
THREE styles of throttle return springs were used. The earliest type fit AROUND the throttle cable center. It was not a good type, and the throttle was pretty stiff. Later types had the springs attached to the throttle lever and a carburetor boss projection with a tiny hole. If a spring is stretched or misshapen, replace it. The spring must not rub against the carburetor or parts, if it does, change the inner fit to outer, where the cable barrel joins the lever. If you now do not remember...rather a bit above...I put a photo, showing a spring, and text on WHY I installed the spring as I did. Far nicer throttle feel can be had by modifying the oldest style CV carburetors so that the spring is NOT wrapped around the throttle cable outer sheath. Not a simple thing to do correctly.
Hate to repeat myself (??), but....If you remove any jets, etc., that have O-rings, you SHOULD replace those O-rings. I prefer to put a faint smear of silicon (or even petroleum) grease on the O-rings when assembling to help avoid cutting them (the tape over threads helps as does room temperature or a bit above, as opposed to freezing weather). Also I put a truly faint amount of silicone grease on the threads when assembling.
The one jet ASSEMBLY that I always recommend be removed, to clean that area in the carburetor is, in order from the bottom (for your future reference): the central main jet; the washer above that (do not substitute the type of washer, don't leave it out); the so-called mixing tube (brass part, with O-ring and outside threads, with 10mm hex sides) above that; the needle jet above that; and the atomizer above that. This information is for those who have removed things, have not 'seen' the direction of assembly, or have forgotten the direction of the parts. Note previous cautions about screwing in the central jet, not to trap and crush the needle, and note also that the atomizer (the top most part) fits only one way, and it will then, properly, stick upwards INTO the throat. You have to have that atomizer rather perfectly vertical to get it into the carburetor body hole.
The needle jet, which looks like a machined brass tube of two basic diameters, marked with a number,
such as 2.64, 2.66 or 2.68, fits with the small tube portion upwards and its slightly curved [internally]
end downwards. Above that part is the atomizer, which is a machined brass part of three differing
diameters, the slightly
smaller diameter goes upwards and fits through AND INTO, the carburetor
venturi (throat), and its lower portion has the holes. On RARE
occasions this part might not seem to
fit and does not seem to want to poke up through the carburetor into the venturi. If the smaller diameter
end is up, the side-holed end down, this is correct, and you may have the part slightly tilted,
and under a tad of too much pressure from the 10mm wrench area below. You can install that atomizer
by itself, and hold it IN the throat, perhaps using a toothpick to get it properly into position and then
fingers to hold it in the throat, as you assemble the lower parts.
You MUST assemble the central jet assembly parts in the correct order. Do not over-tighten.
The central jet assembly O-ring, with the faint smear of grease, will reduce friction, and you
should be relatively gentle on the force you use on the 10mm wrench, just barely tight. Antiseize
or silicone grease on the brass threads make for less corrosion and seizing possibilities.
Remember that the carburetor body is made of a soft and not overly strong material....zinc
probably. The danger is in cracking that central carburetor boss. I grease the threads VERY
lightly, silicone or anti-seize. Those that over-tighten the central jet assembly, and fail to use a
lubricant or anti-seize, are asking for seizure, years later.
Cleaning the idle system...jet, holes, etc...is important, and the Berryman's works well, if allowed to work awhile. DON'T bugger the end of the idle jet with a poorly fitting screwdriver. Rarely the idle jet has been known to freeze in its threads. USUALLY you can use a bit of heat, after an overnight soak of the jet area (vertical) with a penetrating solvent. If you happen to break off half of the screwdriver slot end, you can drill it carefully and use something like an EZ-out. I will use heat on the surrounding alloy, to try to enable removing, before going to drilling.
When replacing rubber O-rings which you must push over threads, you might have
the thought that the threads might cut the O-ring...yes, they could. The slightest
cut will eventually expand to a wide cut or break, and leak. A simple way of avoiding
that possibility is to not only grease the parts with a very THIN layer of silicon grease....but to FIRST wrap a single layer of any sort of tape around the threaded part, and
THEN slide the O-ring over it! Try plumber's
Teflon tape. I use my smooth thumbnails to push the O-ring into
position, sometimes using a teeny SMOOTH, totally NOT SHARP, dental hook,
especially for the last bit of movement into the groove for it.
Don't fail to spray clean the jet in the 'well' of the bowl, and the overflow tube too. I've sometimes had to poke around at that jet with a piece of wire to clean it.
The left and right carburetor bowls are NOT the same. The corner well, containing the jet at the bottom fits the tiny diameter tube projecting downward from the carburetor body. This is the enrichener fuel source.
Except for shop-type overhauls using a heated strong chemical dip tank, I always clean the various metal parts with the mentioned Berryman's brand of carburetor/choke cleaner spray. For a major overhaul, take everything apart and use a professional cleaning tank, especially a heated one, if you have one available. There are many types of spray cleaners available at your local auto parts store. Some are NOT very good. A good one will instantly dissolve a fair portion of the brown stain deposits if you have some, on the carburetor outsides, sometimes a cotton swab will help. I prefer the spray by Berryman, called B-12 Chemtool, Carburetor & Choke Cleaner. This is nasty stuff. Hopefully Government regulations will allow it to continue to have the strong formula. Use outdoors, or with your garage door open! It is also very good in just spraying off the stains on the carburetor outsides now and then. Common so-called 'brake cleaners' are AWFULLY BAD at cleaning carburetors properly. I use Q-tips, tiny bits of cloth on forceps, toothbrushes, other solvents, etc., to REALLY get everything spotlessly clean, inside and outside.
Be sure the main jets, needle jets, and idle pilot jets, all have the proper and same size number on them, left and right.
Rather commonly someone asks about removing, or changing the setting of the slide needles. Even when there is a definite reason, such as you want to richen to the British specifications; changing the needle position by one notch is often WAY too much, and the better method is to change the needle jet to the next size. In many instances the UK-shipped carburetors have both needle size and needle jet size BOTH changed. Another approach is to ASK BING!...they are usually quite helpful...and their carburetor parts and service booklet (with a chart of jetting!) is worth the $.
When installing or removing a slide needle, the early common type of needle is the push-pull/turn type. For them, the proper method is to clean them and your fingers (use piece of suede leather if you have to), so they both are clean and dry, and grip the needle tightly with thumb and forefinger, and rotate left or right, pulling slightly downwards or pushing upwards. EACH 90 degree rotation will allow ONE needle notch of change (if also pushing or pulling). UNDER NO CIRCUMSTANCES BEND THE NEEDLE. Rotate one direction, then the other, as required, with a bit of bias up and down, as case may be. NEVER use pliers!!! ONE (1) 90 degree rotation per so-called needle position!
Be sure both carburetors have the needle sticking out of the slide the same amount. Use a caliper, and measure closely. ***I HIGHLY recommend that you check the slides/needles for distance, to be sure they are the same, as well as to write down the distance, BEFORE removing or installing or changing the needle position. You cannot 'see' the needle position on the early style, it is done by feel, and having a measurement (you need to be accurate to maybe +- .015") may save you some considerable distress. I measure them with a common vernier caliper. You can measure the underside of slide-to-tip distance, or, the distance from top of slide assembly 'tube' to needle tip. Needle position (there are typically 4 positions available) is measured from the top slot position of the needle. Most needles are in the #2 or #3 position from the top.
The other, later type of needle is not held in by the hidden clip in the same manner. There is a screw in the top of the slide. Remove the central screw from inside the slide, & turn the slide over. The needle should fall out the top into your hand, with a little clip on it at one of the four positioning slots. Later needles may be aluminum, & if the clip grooves are worn enough that the needle can move up and down with reference with reference to the clip, replace that needle! The SLIDE needles should be replaced at the 60,000 mile intervals I recommend, or a bit sooner, & always with the associated needle jet. The cost of the parts is FAR outweighed by the excess fuel costs if you do not. There is a particular problem with the aluminum or similar needle material on some 1985+ models, in 32 mm size. There may be some other carburetors with this problem. The NEEDLE grooves WEAR, due to the type of metal used. If the groove wears enough, the needle will not deliver the properly metered fuel. Due to how the carburetors operate, you can have BOTH leanness AND richness, at times. Watch for this, it is little known. Replace the needle & the clip! You can get weird symptoms, even backfiring, if things wear enough. Check BOTH carbs!...they do not, for whatever reason, wear at the same rate.
The carburetors should be mounted squarely to the motorcycle. View from the top, and a few feet
to the rear of the bike. Do NOT tilt the tops towards, or away, from the motorcycle.
The rubber 'hose' adaptors from the carburetor to the cylinder heads are sometimes found to be leaking. This is usually proven by spraying them with any spray cleaner such as 'brake cleaner' at idle speed...should be no idle speed change. Keep the screws on the band-clamps tight. This is the perfect time to spray the throttle shafts too. NO idle speed change at all is proper and the only thing acceptable.
CARB TOP STAINS
(and lean mid-range mixtures SOMEtimes)...
and leaky top plugs:
Fairly often I see carburetor top stains where they join the body of the carburetor. The tell-tale
sign is a brown (usually) stain around the diaphragm joint interface, caused by some tiny gasoline
weepage. I was never bothered by this, as the 'problem' is sporadic and minuscule. However, Oak
sent me (in 1984!!) a bulletin he made up describing this situation as not necessarily being
caused by the lack of the diaphragm acting as a seal, but rather that the compression of that
diaphragm was insufficient for a COMPLETE sealing. He recommended removing the carburetor
and flat sanding the carburetor top itself, with 220 grit wet type paper, kept wet with water, figure
eights, carefully, until the groove, which he said was 0.155 to 0.156 inch deep in the troublesome
carbs, is reduced by about .007 inch. He said to shoot for a final depth of about 0.147 to 0.150.
Remove all grit. I have done this to several carburetors, and it DOES stop the staining. Frankly,
few of you have the gauges to measure this, so you could try just a few
figure eights. TEN is
appropriate, LIGHT medium pressure. Do them EVENLY. Once the fresh sanded surfaces are
evenly fresh all the way around, that is likely enough. Do not overdo this. Clean and reassemble.
Some BMW/Bing diaphragms are a bit thinner than many years ago. If the groove in the top of the carburetor body, where the top casting meets the diaphragm & seals the diaphragm to the carb body, is too deep, then there may be leaks. This has happened even with the correct thicker diaphragms (see prior paragraph). The result may or may not be dried fuel stains on the top of the body, but may also, especially with the thinner diaphragms, be revealed as a LEAN RUNNING CONDITION, perhaps in the mid-range. You can, if experienced, 'feel' the leak by moving the slide up and down with your fingertip, carburetor fully assembled. If you have MIDRANGE lean mixture surging, and it is fixed by moving the needle up one click, then: Remove the top and diaphragm/slide, and sand the carb body until the groove depth is ~0.146".
Measuring of groove depth can be made by various simple means, including using a moderately sharp point on a depth micrometer, or whatever method you prefer..........or, just estimate it all.
LEAKY TOP PLUGS:
Some carburetors (not the flat top ones) have leaky steel plugs on the domes...that shiny center area. You can easily test the plugs for leaks when the domes are off for servicing the carburetors. DO NOT allow any leaks...it will act like a torn diaphragm. They can be crimped or epoxied. I prefer crimping with a tiny tip punch and then epoxying. Some folks have used a toothpick and aluminum paint. If your plug has a letter C stamped into it, do not cover it up, it means something....it identifies an early modification in the /5 era. If you'd like to, assuming no leaking dome top plugs, there is a BMW Roundel that can be epoxied to the top of the carburetors (they fit the small top ones, also the flat top ones, which can also use much larger ones too).
See http://bmwmotorcycletech.info/hardware.htm article on this website, for a huge list of Roundels, including exactly which fit nicely the small tops. When cleaning the area for epoxy or aluminum paint, etc., it is very difficult to get sandpaper into the corner. I use a dental pick or awl.
Maybe ONE carb does not work well initially, then works OK after engine running & throttle opened.
It was in November of 1971 that BMW first & almost lastly; recommended OPENING the throttle during 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.
If starting is poor, and you KNOW the enrichener items ARE 100% proper, and KNOW that the valves are set correctly, and KNOW that any ignition points ARE set properly....then...it may be that the slides are not returning fully. Older advice was to check for that and fix by installing springs 13-11-1-335-324 above the slides on 40 mm carbs, and 13-11-1-338-134 on 32 mm carbs. This did not apply to the flat top carbs.
Tom Cutter once said 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 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 DO 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.
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 (maybe even until engine warms a bit) and throttle is opened.....check that the enrichener was properly installed, no matter if the dot-dimple on the shaft looks correct.....and see if the butterflies were properly installed. If the butterflies are reversed, or do not align in the carburetor throat well, you will never get proper operation.
If you have a R75/5 that is particularly a bear to start (or, one carb is not working at startup), and everything else checks out fine, be SURE to check the slides, to be SURE they are BOTTOMING fully, and not hung up slightly. Install the springs above the slides. This problem is rare, but has been seen on them, and VERY rarely on later carbs. Polishing the slides and inside carb body can help. There is a very complete article on this website about R75/5 carburetor problems: http://bmwmotorcycletech.info/earlybingR75CV.htm
ON all models, and the Independent float KITS too;....the floats are adjusted by bending the tab that the float needle (and wire clip, if used) attach to. I have found a TINY screwdriver does this OK, and seems to work much better than unwieldy long nose pliers. Do one carburetor at a time. BE GENTLE AND CAREFUL!!...do it evenly too...not at one edge, which bends the tang on an angle you don't want. In other words, keep the tang flat and square. It is spring-y, so eyeball your work. After making a small change... turn gas on, lift float gently and SLOWLY with protectively gloved finger, until the gas flow JUST stops. At THAT point where the gasoline JUST stops flowing, the top of the stock floats are to be parallel to the lower body of the carburetor. I allow as much as +- .020 inch. I have done some fine-tuning by playing with the float level, I suggest you do NOT. Some folks...including me... find it best to lift the float until the gas stops flowing; then very very gently lower it until the gas JUST flows. This IS MY PREFERRED METHOD as it eliminates effects of the spring loaded lower tip on the later type float needles & any excessive existing play in the pin & hinge holding the floats in the carburetor. For the KITS, the adjustment is the same, but the measured distance is .412" from either lower arm edge to the body. I have information on this website ....originals of some Bing documents, some of marked-up copies, crude, but useable ...all of various Bings sheets on installation and adjustments of the independent floats KITS. Here is a hyperlink: http://bmwmotorcycletech.info/bingindependent.htm
Be gentle & careful about doing any bending. Float level affects richness-leanness & gas mileage.
Major tuning/adjusting the carburetors is not part of this article.
HINT: On earliest models the band clamp at the carburetor throat inlet should have its adjustment at the TOP...or, at least not at the bottom. The models most affected are those with the plastic tube having slots in the band-clamp area. If the band adjustment is not above the bottom, the breather output oil may drip on your foot, depending on if there is a slit or not on the tube.
Additional information will be found at http://bmwmotorcycletech.info/bingcv-2.htm
to 12-07-2002: mostly clarifications.
Through 12-09-2002: add area for upcoming float testing; added references to Bing CV Carburetors-2, many clarifications and emphasis items. This version was not to be released to the website, pending more additions.
01-30-2003: Updated many places, decided to release to the website even though float testing has not quite begun yet. Added a complete section on the enrichener orientations.
03-30-2003: extensive information on Bing dual independent floats added.
06-21-2003: Add NOTE and HINT on band clamps.
07-13-2003: Add all information on float testing done; clarify many areas.
07-14-2003: Edit for clarity
07-20-2003: Add note on float/float pin clips not being used on some models
07-22-2003: Expand about the two types of retaining for the slide needles
09-30-2003: add top of article notes on Gary L. Smith's article
09-30-2004: add url for Bing's chart of component description and numbers
02-15-2005: minor updates.
02-18-2005: hyperlink to bingindependent.htm
10/18/2005: general updating
11/30/2005: update enrichener/choke information
04/23/2006: add emphasis note on screwdrivers for the Bing tops. 04/24, modify that again
05/11/2006: Bing diaphragm caution note
08/24/2006: Add photos of enrichener parts/orientation (left parts)
03/04/2007: more information on the Stromberg diaphragms and cautions on their use.
05/03/2007: Fix hyperlink to BMWMOA.org; and generally update the entire article
07/06/2008: replace enrichener photos with better ones.
04/19/2010: Expand description of the enrichener parts assembly and alignment, regarding the dimple, jet disc, etc.; later in evening, add more comments, so is no mistake about how the enricheners all assemble.
04/25/2010: Add photos, Posi-Drive screws and screwdriver tip
04/26/2010: Twice today go through the entire article, simplifying, changing to more legible & less numbers of fonts & colors. Especially, fix the Enrichener section, making clarifications, adding photos, etc. Final version release 3:12 PM PCST
02/16/2011: Added another view of enrichener cap assembly...with commentary.
05/15/2011: Clarify the enrichener a tad more, and remove one photo that was confusing.
08/15/2011: Revise diaphragm and Stromberg information
05/14/2012: add butterfly photos
05/17/2012: Clarify information on the slide springs.
10/14/2012: QR code and google update code
11/06/2012: Expand Bing independent adjustment area
10/09/2013: Add more to information on float needle and seat.
07/15/2015: Update a few areas for increased understanding/clarity.
09/03/2015: Add photo of carburetor with control lever springs and orientation. Clarify some details in various text areas.
09/05/2015: Add more photos and text, and clean up some. Finish-up, and reduce colors, etc., on 09/07/2015
09/12/2015: Remove the long section on float testing, and move it to a new article. Reduce colors/fonts, clean-up.
11/22/2015: Add note on -812 O-ring, to show discrepancy between specification and actual measurement.
01/06/2016: Update meta-coding; begin narrowing article; left justification; horizontal line % changes, font increased, etc.
05/08/2016 Remove entire table layout in favor of no table. Revise layout 100%. Minor clarifications.
© copyright, 2014, R. Fleischer
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Last check/edit: Sunday, May 08, 2016