Bing CV Carburetors

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

10. If your bike tends to stumble and run out of power after a good uphill climb or a higher speed run down the highway (this could happen quicker with a very full tank), loosen the gas tank cap...if there is a whooshing noise and the bike runs well again after waiting 15 seconds or so, it is the tank venting that is clogged. Similar problems with fuel flow can come from clogged in-tank or in-petcock filters, or clogged aftermarket filters.

11. Most petcock models (there are at least 5 models), if they get stiff, can be taken apart, cleaned and silicone-lubed. Earlier versions of two petcock styles had a tall screen that stuck up inside the tank, that screen used a separate reddish-brown cambric washer. Remove washer and screen and replace with the later one-piece bonded screen/washer. It not only screens particles better, but seals better. Some petcock models have a screen built into the lower outlet...remove and clean it regularly. Remove and clean and dry out the tank every year too.

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

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

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

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

16. Diaphragms: Here is a link to information on earliest Triumph Spitfire.....you will have to wade through menus to the fuel system...may still operate: http://www.spitbits.com. Stromberg CD150 ""supposedly"" uses the same diaphragm (B20924??) as the Bing 32mm. For 40 mm, it was CD175. See also 1970's to 1980's Volvo. My personal opinion is that the Stromberg diaphragms are not the same in flexibility, and vary otherwise too...and....are for emergency use only....and you could, if yours gets a rip or tear, simply glue it, or use a glued-piece of condom, or whatever. It is my belief that the Stromberg diaphragms vary greatly with supplier, and if too stiff, you will have LOUSY power availability, and likely an inability to rev high enough. There is a rumor, that I do not know the truth for, that Bing USA is selling the Stromberg diaphragms under their own Bing label. My advice remains exactly the same as it has been for years...do NOT buy ANY Bing parts from Bing USA, UNLESS the parts you need are NOT sold by BMW! Use genuine BMW parts.
The 32 mm overhaul kit from BMW is 13-11-1-258-051
The 40 mm overhaul kit from BMW is 13-11-1-336-902.

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

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

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

Here are photos of the Posi-Drive screw, and the screwdriver tip. Note the differences from a Phillips or a Reed & Prince. Notice, for instance, the nearly flat bottom in the screw and the associated tip of the PosiDrive screwdriver.....note that a Reed & Prince tip is SHARP. Note also that the PosiDrive has partial 'splines' in between each regular size 'spline'. PosiDrive's are NOT designed to slip at the low torque that a Phillips screw IS designed to slip at.

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

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

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

This is an edited version of a reply I did on the Airheads LIST, on November 27, 2009:

Each needle notch is roughly the equivalent of 2-1/2 to 3 steps (2.64; 2.66; 2.68....) in the needle jet. Thus, a needle notch is a relatively coarse adjustment. Needles are made in both different lengths and tapers to match the carburetor and engine requirements; and I absolutely do NOT recommend changing from the stock needle to any other needle number; doing so would open a Pandora's box of problems. Changing the stock clip position or needle jet, is not in that category.

The question came up as to whether or not to change jetting if riding at a mile high. 5280 feet can be MUCH higher than that 5280 feet in "density altitude" in hot weather. The engine and carburetor respond to the actual air density, and density decreases (increase in density altitude) rather fast with temperature (only a little with humidity); and is also affected by the barometric pressure; which is generally not a large effect either.

For every physical map altitude, there is a corresponding temperature on a chart of standard values. For one mile high physically mapped, the equivalent standard temperature on the chart will be approximately 40°F. This means that if you are in an area that is around 5280 feet in elevation, and the air is warmer than 40°, then the altitude that the engine/carburetor thinks it is, is HIGHER. The reverse is true, if the temperature is lower than the 40° standard value.
Example: I live at 6,200 feet. At times, in the Summer, the density altitude can be towards 9,000 feet. People that come here in the summer, and hike into our higher mountain areas, might find themselves quite out of breath....the density altitude can easily exceed 12,000 feet on a warm day, and I think I saw nearly 14,000 once. Your engine will lose power output as you go up in altitude. It is usually quite noticeable by about mile high. It gets much worse, and at a faster rate, as you go above 8,000. You will also likely notice that your idle rpm has dropped, annoyingly perhaps. The power output DEcrease is offset by carburetion jetting (main jet change and maybe needle jet change) modifications by only a modest amount.

If the fuel level in the float bowl is correctly adjusted; and the engine is stock, and the air cleaner is not clogged; needle and needle jet not worn;....ETC....then the STOCK jetting is generally close enough, and SAFE ENOUGH; for BOTH sea level and anything reasonably higher. "Jetting" here means main jet and needle type and position and needle jet number.

As you go UP in altitude, the carburetor will operate richer. Whilst this might burn more fuel than necessary; and ultimately carbon-up the combustion chamber more, it is SAFE. If you were to use a leaner "jetting" ("jetting" used in its broad sense here), power might come up a bit; and that effect is more and more as you go further up in altitude, particularly above 7,000 feet. But, the problem is that if you change the jetting by more than a rather modest amount leaner for riding at high altitude..., then as you go down in altitude, say to sea level, you may be running too lean, and could possibly burn a hole in the piston if extreme enough.

Thus, unless you plan to be exclusively riding at around a mile high or above, I'd recommending you NOT change the jetting from stock.

If you were going into very high elevations, perhaps very consistently over 8,000 feet (a rough guess on my part here), then I would lower the main jet number, by 5 points (more if over 10,000 feet); and I'd also lean the needle jet or needle position. Be very wary of going to lower elevations, so change your 'jetting' back before doing so.

The Bing CV carburetor itself tends to compensate for elevation changes relatively well, except for the nearly or actual wide open throttle position (where the MAIN jet is almost 100% controlling the jetting).... and I would not expect too much of an improvement by changing jetting, for only a change up to perhaps 6000 feet.

The Bing CV carburetor needle and needle jet operate primarily in the mid-range of throttle position, that is, roughly from 1/4th to 3/4 throttle, ...with some lesser effects lower and higher. The needle and needle jet have very little effect at full throttle; and only a small effect below 1/8th throttle.

Bing publishes a roughly 8" x 11" booklet on their carburetors, that has a LOT of information in it. The booklet covers all the models by number, parts numbers used for that model, and extensive information on how the carburetors operate, and are tuned, in case you might want to make changes. You can actually calculate main jet numbers from the information given. BUT, Bing has NOT put any real information on tuning the needle nor needle jet, for altitude, into that book. I recommend purchasing the book if you like having such things.

What I've posted above is not to be taken as all-encompassing.

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

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