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Carburetors, as used on BMW Airhead Motorcycles

 

Some time ago a very embarrassed Airhead owner contacted me about how to 'fix' his carburetors. He has Bing CV carbs, and replaced the diaphragms, floats, etc. himself. Frankly, he is a bit ham-fisted. He also hates to ask for advice. He proceeded to nearly destroy his carburetors. The many e-mails with him prompted this article. There is a LOT of information in this article, and lots more on the author's website. Carburetor work is not very difficult, easy to learn, and once you have worked on your carburetors, perhaps at such at an Airhead TechDay, you will wonder what all the fuss was about and you will find that it is easy the second time, and you might hardly need to refer to this article ...or one of my others on carburetors...again.

I have my own ideas about when one should consider working on your Bing CV carburetors (and fuel tank and petcocks!):

YEARLY: depending on mileage and use: clean/dump float bowl contents, check corner bowl jet to be sure it is clear/open, possibly replace float bowl gasket if poor; empty gas tank, remove petcock(s), clean tank, dry it, clean petcock screen(s), possibly service petcocks internally if they are getting quite stiff to turn; or, if leaking.   Inspect fuel hoses. I consider these things part of carburetor servicing.  Failure to service the fuel tank will eventually be costly.

5,000 mile intervals: synchronize the carburetors, but ONLY AFTER the valve adjustment and ignition timing adjustments are checked FIRST.

30,000 mile intervals: replace float assembly (if original one piece assembly).  Always also replace the float needle and clean out the central jet assembly.  Replacing the float assembly is UNlikely if you have the alcohol-proof separate floats that Bing now offers.  However, if you have those, be sure to replace the float needle and 

{mprestriction ids="4,5,6,8,9,10,11,12"} check the adjustment of the flimsy brass floats bridge, info on the Snowbum website in its own article on these independent float kits:
http://bmwmotorcycletech.info/bingindependent.htm

 

60,000 mile intervals: replace diaphragm, slide needle & needle jet.   If you have the ALUMINUM slide needle, you may need to replace it and its jet more often.  The aluminum needle notch tends to wear, besides the needle shank and jet, and, opposite to the steel needles, the notch (clip) wear tends to cause LEANNESS.

Upon condition:  Disassemble and overhaul (removing butterfly valve and replacing the shaft rubber O-ring may or may not be done, your choice, do upon need typically)


Bing sells parts, individually and in various rebuild kits, and has a useful manual available, and can make recommendations on jetting changes for slightly better performance, as can some folks, including this author, on the Airheads LIST.   You can purchase MOST parts and rebuild kits at your BMW motorcycle dealership; but they offer only the standard stock parts.  BMW dealerships are OFTEN MUCH CHEAPER than direct from Bing!  Bing also offers a vacuum take-off kit if your carburetor does not have such. Bing website: www.bingcarburetor.com

The original type of stock dual one-piece WHITE plastic float assembly does NOT always fail by getting heavier, and they do fail, slowly, over time. Sometimes there are changes to them that are more subtle than a weight change. It appears that alcohol is not the only factor, and I have been UNable to prove that ethanol IS 'the' problem.....probably other oxygenates, etc., in modern pollution-controlling gasoline's are the over-all cause of float deterioration.   A bad float WILL sink in gasoline, essentially less than a third, or, no part of it, will be above the surface in a container of gasoline in which the float, without its holding pin, is trying to float. Once it deteriorates and begins to sink (so less than approximately a third is above the fuel), your fuel mileage will get worse and worse; and eventually the carburetor will leak all over your boot (this is another reason from the usual, which is dirt in the float needle area). I did a big study on dozens of 'bad floats' and new floats, quite some time ago. This included long term soaking in various gasoline's AND components of gasoline's, dimensional measuring, weighing, taking apart....etc.  UNfortunately I did NOT come up with any particular solvents, etc., that were proven to be responsible...and my tests were extensive, over many months.  MY GUESS is that tests would need to be over YEARS, to be definitive.  I summarized the tests in one of the carburetor articles on the Snowbum website.

Bing sells a  'Alcohol-Proof Float Kit'.  This includes float bowls, and a special brass-looking float bridge that the floats operate against.  The floats are SEPARATE and independent of each other.  These kits are expensive, do give seemingly unlimited float life, but don't help much with anything else, which is contrary to Bing advertising. This includs fuel mileage and better mixtures in cornering. ONE exception might be for truly VERY aggressive riding in twisties.  These kits have some serious drawbacks.   I do NOT recommend Bings so-called alcohol-proof float kits. HOWEVER....many are in use and mostly there are NO problems, to be honest about this. Bing began selling these kits many years ago. BMW NEVER installed them in the airheads...they had reasons not to, and $ was hardly the reason. Bing's original engineering/sales literature claimed all sorts of benefits, such as mileage and throttle performance improvement.  It is my belief, backed up by old Bing engineering literature, that the origination of these kits (particularly the independent floats portion of the kit, rather than the float material itself) was for use in aerobatic home-built airplanes, in which there would be definite advantages at 'unusual' attitudes....that you hardly will experience on a motorcycle particularly with the layout of the Airheads, and questionable even in racing.  I have seen no documented mileage or throttle performance improvements from swapping "known good original style floats" with the Kit parts.  Mileage could certainly INcrease if the stock old floats were faulty or the float adjustments were not proper.   Once these PRICEY kits are installed, and operating correctly, you can likely forget about replacing the floats again (not so for float needle, which is the same as the stock needle). These independent floats kits consist of, for each carburetor, two black floats, a brass-looking flimsy sheet metal bridge, and a special lower bowl.  The kits installation eliminates the overflow tube (or, call it the bowl vent), that in the stock bowls was a tiny thin tube that stuck up from the bowl bottom.  With that missing, there is no allowance for an overflow from a leaking float needle, a COMMON situation, and usually from either a very well worn old float needle, or a microscopic bit of dirt in it or on the needle seat. The carburetor bodies have a vent, that can also act as a higher-up overflow.   Unfortunately, and especially with the left carburetor, if the bike is on the sidestand, fuel MIGHT then flow into the cylinder.  That could cause major engine damage upon trying to start the engine. The answer to that possibility is to ALWAYS...ALWAYS!!....turn off the petcock(s) if you use the sidestand.   For an extra measure of safety, ALWAYS turn off the petcocks when parked, sidestand or centerstand.

The stock bowl tubes, not available in the Kits, have rarely leaked/weeped, at the carburetor bowl base and if so leaking, they can be lightly tapped at the base by a tapered punch and/or sealed by epoxy or similar substance.   I do both.  Rarely that tube has cracked, and that is repairable.  I think most cracking comes from freezing water in the bowl, would not happen if bowls were emptied now and then.

So, emphasizing here:   there is no overflow tube in Bing's 'Kit', Bing said it did not have room for one in the kit bowl (I think they COULD have), so, theoretically, if a float needle should fail to seat (that is pretty common too), fuel could continue to flow into the carburetor, and past it, and fill a cylinder...a VERY BAD thing to have happen if you try to start the engine, you could bend a rod or have worse problems. In practice, this cylinder-filling does not seem to be happen much, and mostly any overflow is coming out the carburetor body vent. Theoretically, the tendency might be MUCH higher for the left cylinder, as bikes are often parked on the sidestand. Be aware that it could happen. The second problem I have with these kits (notice that BMW never installed them!) is that they have a IMproperly designed material for the flimsy 'brass-bridge'....and it is certainly not heated treated to be stiff enough. Thus, Bing's kit has instructions on how to adjust this bridge.....and recommends you adjust it a second time, later on. I find that SOMEtimes this bridge is warped, the floats may have an interference problem, and that the adjustment may continue to shift for awhile. In adjusting the bridge, I do it per Bing's instructions, and then I measure the ACTUAL fuel level in the bowl. I recheck that a few times over the next year or two....although they usually stabilize after a month or two of riding, and one or two adjustments after the first one.

There are similarities between those Bing Alcohol-Proof kits, and the stock setup, and the following assumes the stock setup, but you should have NO problems understanding any of this, if you have those kits, as the ONLY difference will be the adjustment of the floats fuel level.

NOTE!   Early 'kits' had dark colored plastic bowls, and the corner jet came separately, was to be installed into the bowl by the person assembling the kit into the carburetor.  Be sure your corner jet is in the correct corner!...that corner has a thin round tube coming DOWNward from the carburetor body.  The left and right carburetors are REVERSED in that corner jet placement.   Latest Bing kits should have the zinc metal bowls, which do not exhibit a problem that some of the plastic bowls did:  microscopic cracks, often at a corner, that caused weeping.  The plastic bowls are, to my knowledge, UNrepairable by ANY solvent or glue or epoxy.  The metal bowls are pricey....and BMW offers only the stock metal bowls, not the KIT type bowls, so if you need a kit bowl, or independent floats, or their specific type of bridge, you must go to Bing Agency.

Your stock floats are held into the carburetor by a pin that is knurled on one end. Be SURE to push/tap the pin out in the proper direction!! ....That means pushing it out from the non-knurled side of the pin, and reinstalling it from the non-knurled side. When removing the pin, use a very small round tool, even a ground nail, and be careful, you will RUIN your day if you break the ear off the carburetor.   When reinstalling that pin, this is NOT the time to use Pliers, ViceGrips, or ChannelLocks, and break the ears (float pin bosses). On most Bing CV carburetors, the floats are tied to the float needle with an EASY TO LOSE small and exceedingly fine wire clip that ensures positive needle movement with the floats. Some call this wire clip a 'hairclip'.  On those carburetors with the clip, the float needle lower end has sort of a tiny plunger and a tiny hole is in it that the wire clip fastens to, and this end plunger part rotates easily.  It is an annoyance re-assembling these. When installing the float, float pin, float needle and that tiny wire clip, they must be installed as an assembly, a bit tricky, especially if the carburetor is right side up, in normal position on the bike...but doable. I suggest a white sheet under the carbs, in case you 'lose' that wire clip or needle. I HIGHLY recommend that you order SEVERAL spare wire clips when you order floats and float needles.   ALWAYS replace the float needles when replacing floats; when they leak your boot gets wet, the mixture gets very rich, and the fuel mileage decreases considerably.

NOTE: It is fairly common to have a carburetor flood-over, and wet your boot. MOST of the time this is due to some sort of very teensy particle of some sort of dirt/etc., that passed through the fuel tank petcock screen, and lodged itself in the float needle tip area. Deteriorating fuel hoses also are a cause!  The usual road-side 'fix' is to turn the gas off, remove the float bowl (be careful with the gasket), then turn the gas on, and wiggle the float up and down to shut off and on the fuel flow... but not forcefully, do it a time or two, then turn gas off.   Empty the bowl of its contents if there are globules of water or other contaminants seen.   Reinstall the bowl...again being careful of the gasket.  The thick wire clip that holds the bowl to the carburetor must fit into the carburetor correctly....if it is spread wider, it can come out of the carburetor and you will lose the bowl, so reform the clip if it does not fit the body properly.  That is rare, but has happened.

 Due to the aging of our fuel tanks interiors, I HIGHLY recommend you install a fuel filter downstream (below) for each petcock. Use paper element or scintered element types from your nearby autoparts store. If you have continuing flooding problems, even with such filters installed, you either have deteriorating rubber fuel line hoses....or....you need a float needle change, or you have a bad float.

It is possible for the float needle SEAT to wear out. I have seen this VERY RARLEY, all were on carburetors that had water in them, said water either contacted that seat, or the vapor did, over a very long period of time...in storage. The float needle seat CAN be replaced. It is a tad tricky, and an Eazy-Out can be used Information on how to do it is on my website: http://bmwmotorcycletech.info. Consult with Bing on availability of the seat. In some instances, I resurfaced the seat face, in situ.

NOTE that the SLIDE needle; especially the later alloy types, as opposed to early steel types (which also wear, but much more slowly); and, the associated needle jet, wear from normal use....the needle is purposely loosely mounted, to wiggle around inside the jet. If you do not replace these needles and needle jets at a reasonable interval, about the maximum is 60,000 miles, your fuel mileage will continue to slowly deteriorate, due to an increase in mid-range richness. I recommend that no matter the type, you replace them at NOT OVER 60,000 miles.   The aluminum needles wear very badly, and not only do the needle portions that fit the jet wear....but the 'clip position' wears BADLY.  Replace those needles if the needle has a lot of up and down movement when installed.  The 'notch' wear is very apparent when looking at the needle.

I recommend that the diaphragms be replaced at around 60,000 miles. Since the kit containing the two diaphragms and O-rings and gaskets are about $40.00 or more from BMW, I have NO objection if you let the diaphragms go until they fail by getting a hole in them. You can purchase the individual parts too, of course. Diaphragms seem to hold up rather well over TIME, so I cannot give a replacement period, just for time alone. Most diaphragms have a downward facing tab that fits into a small recess in the slide itself and a somewhat larger tab that faces downward that fits in a corresponding slot in the top of the main carburetor body. These 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 slide does not go fully downward easily when placed into the carburetor, NO pressure is needed!...then withdraw the slide and try again...the needle might be hanging up on the lower jet part.    If the diaphragm assembly is held with 4 screws, tighten evenly. There is a version of the carburetors using a plastic ring for the diaphragm, do not break it, and a bit of heat from a hair-drier or in hot water, will make it less brittle; less brittle means you won't be likely to break it.

When the main central jets and such are all re-assembled to the carburetor, and you are ready to install the slide/needle assembly, be gentle, some care and wiggling is sometimes necessary to get the needle into the lower brass tube area. Slides are reinstalled into the carburetor clean and dry.  The lower jet assemblies that the slide needle fits down into, really should ALREADY be in the carburetor, that being the safest situation.   If you assemble the lower jet units AFTER the slide/needle is installed, be VERY careful that the needle does not hang up on the lower jet (atomizer part). DO NOT use much force at all on the central jet assembly (10 mm wrench). Be carefull!  You do NOT want to tighten up the lower jet assembly against a needle tip; nor, have the atomizer hanging up in the carburetor.  That part has only ONE end that WILL fit into the carburetor throat. I will get into this a couple of paragraphs further on.

When assembled correctly, the slide, which has two holes at the bottom, off center, will have those two holes facing the cylinder head.

***The following item is CRITICAL, and is where my fellow Airhead owner REALLY messed up, although he certainly made a mess of some other things. When you install the main jet, and the parts above it, which I refer to as the Central Jet Assembly, you should install these parts BEFORE reinstalling the slide/needle/diaphragm assembly. If you are careful, and wiggle the slide needle, and watch things, this is not a necessity nor problem....but....failure to follow my advice can lead to bending the slide needle (at a minimum) unless you are careful. You can also cause a real hangup 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 the carburetor body ..$$$. Sometimes a sleeve is made and installed, perhaps epoxied. A new carburetor body is REALLY expensive ...unless you find a used carburetor from a wrecked bike. Be careful! The reason for the caution is that the lower assembly MIGHT hang-up on the needle. Not a problem if you wiggle the needle during central jet unit installation (simply put another finger into the intake area, guide the needle into the lower atomizer jet area).  There have been folks who assembled the very top part item of the Central Jet Assembly (the Atomizer) upside down...well, tried to; or, failed to see to it that it's proper end fit into the throat.  Be careful!  This takes many more words, than just doing it correctly, which takes a few seconds.  That atomizer, at first glance, looks like either end will fit up into the carburetor hole and thus stick up into the venturi, but the atomizer ends are NOT the same diameter, only ONE end fits upwards into the carburetor hole.

When one does a mini field-overhaul on a Bing CV carburetor, it is usually NOT necessary to totally disassemble the carburetor.  My e-mailing Airhead (I am trying hard not to type that with a small 'a') removed EVERYthing! Normally, one needs only to replace a diaphragm, a float, and a float needle, and do some spray cleaning. If the idle mixture screw is removed, replacing its O-ring is a good idea. One CAN remove only the necessary parts, then spray into all the jets (pilot jet, bowl jet, central main jet assembly) and holes with a strong carburetor spray, and let sit awhile, then spray again in every direction possible through those holes. You might consider spraying ALL the metal pieces, wiping with a clean cloth. If you remove the central jet assembly and the idle mixture screws, it is a GOOD idea to replace the rubber O-rings on them. Wrap the metal part with a layer of electrical tape during sliding-on the rubber-O-ring, to avoid threads cutting the O-ring....and I like to use a FAINT amount of silicone grease on any rubber O-ring.  The grease sold as Dielectric Grease at autoparts stores is usually a silicone grease.  O-rings seem to last longer, have less chance of being cut by threads, etc. if you use such grease, use ONLY the faintest amount. DO NOT GOOP THINGS UP!

***WARNING!  nearly every year I hear of someone breaking off ~half of the screw slotted area of a pilot jet.  Be SURE your screwdrivers are the correct size and fit well!  Do NOT force things.   Application of some heat MIGHT help with a frozen jet, if not, add a penetrating oil and soak a day or three.  Jets need NOT be installed overly tight.  Some use a FAINT amount of silicone grease on the threads and rubber O-rings.  Some use a FAINT amount of antiseize compound on the brass jet parts threads.

NOTE...repeating myself here!   The atomizer that fits into the bottom of the carburetor throat, from below, sticks up a bit into the carburetor throat.  That part has UNequal diameter ends...only one end fits upwards into the carb...sometimes you have to help it go straight up and into the carb throat...I use some sort of tiny tool, metal pick, or toothpick.   It is easy to put central jet parts in wrongly. Be SURE you do NOT do that.

Some Bing slide type carbs use an acceleration jet assembly in the central assembly, these parts all come out mostly at one time, same as those carburetors without. Cleaning with spray solvent is all that is normally required.

Back to the CV carbs:
Sometimes the brass part that sticks upwards into the carburetor throat (atomizer) does not fall downwards and out when the central jet assembly is removed. Use a toothpick or similar to gently dislodge it. NEVER clean jets with tiny drills, pieces of wire, etc....unless you are exceedingly careful not to enlarge the jet. Usually, a very strong spray solvent as recommended in this article will do the job, if used a few times over a couple of days, in very stubborn cases of lacquers deposits from fuel.

It is often a waste of time to disassemble the enrichener (often called a choke, which it is not)...and the enrichener pieces are EASY to mix up and get installed backwards. Especially left and right. If you are doing an overhaul, you should remove all these parts, one carburetor at a time, and pay attention to how they fit. The enrichener (choke) is held to the carburetor body by 4 phillips head screws. All carburetors after the earliest ones use an enrichener cover gasket....and it MUST be in good condition. These 4 screws are famous for loosening, and thus causing problems. If you take the screws out, clean them and their holes, and use a TINY bit of Loctite BLUE on the threads before tightening. I lubricate the enrichener parts with a faint trace of silicone grease, but I am VERY careful not to get any into the very tiny passageways in the enrichener disc (especially the later types with more and tiny such holes). In the R75/5 era, the Bing enrichener cover had the O-ring on the OUTside, if you have a depressed area there, that is good to know, because many have tried to use the rubber O-ring INside.   NOTE that some enrichener shafts have been WRONGLY PUNCH MARKED by the FACTORY.  See the author's carburetor articles, for photos and description:
http://bmwmotorcycletech.info

In case you have mixed up the parts, or the prior owner did, here is the information on the enricheners: The brass shafts that operate the enrichener are stamped in the inner ends, L and R, for left and right carburetors. The late model enrichener rotating thick metal disc has an elliptical hole, and 4 smaller holes, one of those 4 is a bit larger. The holes MUST be clear. If you have the LEFT enrichener unit off the carburetor, and put it in front of you, upside down...that is...you are facing the inner side... orient it so the round protuberance of the outer casting is to your right, and the lever is upwards to its stop (around 1 O'clock?)....then the elliptical hole of the disc is roughly opposite the upper left casting screw hole, and the 4 tiny disc holes are roughly to the lower right. If you have the RIGHT enrichener off the carburetor, for the SAME orientation of casting and lever, the disc is REVERSED, that is, the 4 holes are to the upper left, elliptical hole is to the lower right. It is VERY tricky to properly install the two metal pieces that make up the LEVER of the enrichener. The tricky part is that it SEEMS that they can be assembled in more than one position(s) that WOULD work OK. Not only can the levers be installed upside down, but also reversed in position. In both Right and Left, the part that has NO notch for the installation of the wire of the barrel/cable inner, goes onto the carburetor enrichener shaft FIRST....with its offset facing the carburetor body. The OUTER part, that DOES have the notch for the cable wire, can be installed wrongly....flipped-over, if you will. Install it such that the notch does NOT face upwards during cable operation.

NOTE that some enrichener shafts have been WRONGLY PUNCH MARKED by the FACTORY.  See the author's two main carburetor articles, for photos and description:
http://bmwmotorcycletech.info

***One should PROBABLY NOT remove the throttle butterfly valve unless there is a good reason. The shaft has a rubber O-ring, the butterfly screws are peened, and the shafts can still be leakproof with a fair amount of side play due to those O-rings being rather effective. To test for leaking, and do this BEFORE you overhaul the carburetor:   Spray shaft areas with brake cleaner while engine is idling. In fact, this spray test is a great one for other potentially leaky areas, such as the carburetor tops, hoses to the cylinder head, etc. If the idle speed changes, you have a leak. If you remove the butterfly to install a new rubber O-ring on the throttle shaft, be SURE to lubricate that O-ring with silicone grease, and be SURE to note how the butterfly came apart...the bottom outward side must remain that way.  Mark it if you have to, such as TOP, OUT.  or?   NOTE that your butterfly could have been wrongly installed previously.  In fact almost anything could have been done wrongly previously.  SEE my carburetor articles on my website for photos of correct and incorrect items, including the butterfly.

Bing may have marked your butterfly with punch prick marks, etc. You will have to grind or otherwise remove the peening, obtain new screws, and align the butterfly with the carburetor body in the closed position with your fingertip, and slowly tighten the screws. You will need something to allow you to repeen the new screws without bending the shaft.....[or, use Loctite BLUE on the cleaned threads].    It is important that the screws NOT be tightened until the butterfly is positioned correctly.  Not only must the butterfly be installed correct side out and up, but the butterfly should be centralized as best possible.  I do this by tightening the screws VERY lightly, and rotating the shaft as I press the butterfly lightly in the closed position, so it fits the venturi/bore as best possible, then I evenly and carefully tighten those BLUE Loctited screws.  There are photos of a correct, and incorrect, butterfly alignment, on the author's website: http://bmwmotorcycletech.info

The enrichener (choke) control and the throttle control on the carburetor each have a return spring, they are not the same type. If either is stretched or misshapen, replace them.  Softer aftermarket springs are available and work OK, and lessen wear on cables and especially the throttle assembly at the handlebars.

The earliest /5 carburetors have stiff throttle control springs around the throttle cable, which makes for very stiff throttle action. These early carburetors do not have the ears for the later type spring mounting, but can be modified to use later type springs if you are clever and handy. Bing MAY be able to offer later cover tops to fit.

Some carburetor models have a large many-coil fine spring above the slide, the purpose of which is to ensure positive slide return.

If you have the troublesome very early R75/5 carburetors, the author has an extensive article on them on his website:  http://bmwmotorcycletech.info/earlybingR75CV.htm

For your future reference, the central jet assembly order of parts, from the lowest part, is as follows: the central main jet; the washer above that (do not substitute for the type of washer); the so-called mixing tube (brass part, with lightly silicone greased rubber O-ring and with outside threads, with 10mm hex sides) above that; the needle jet above that; and the atomizer above that.   The atomizer, as noted previously, has two different diameter ends, only one end fits into the carburetor throat hole.  If you have the acceleration pump assembly...some R60, some R65, some others, ....it is SLIGHTLY different. AGAIN, I caution that when reassembling, use a faint tad of grease on the threads and just replaced O-ring, and assemble BEFORE you install the slide/needle!

NOTE:  The trick to getting rubber O-rings onto parts that have threads that might cut the O-ring during assembly, is to wrap a turn of plumbers Teflon tape over the threads, slide the O-ring into position, remove the tape.  VERY sparingly grease the O-ring during and after assembly.

The needle jet, which looks like a machined brass tube of two basic diameters, and is marked with a number, such as 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 small diameter goes upwards and fits through AND INTO, the carburetor venturi, and its lower portion has the holes. On RARE occasions this last part, the atomizer, 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 (or tilted and under a tad of too much pressure from the 10mm wrench area below). In that case take a toothpick or similar, and GENTLY straighten it while GENTLY screwing the jet assembly upwards. You MUST assemble the parts in the correct order. Do not overtighten the lower jets assembly. The O-ring, with the faint smear of grease, gives a bit of friction, and you should be relatively gentle on the force you use on the 10mm wrench, just barely firm. Remember that the carburetor body is made of a soft and not overly strong material, probably zinc.

It is probably optional about removing the idle jet, although I DO like to do that, so I can use a spray wand more properly, and the mixture adjustment screw as well.  Spraying into the various holes that are available has already likely done a good enough job, but if you do, and I recommend it, you MUST replace the O-ring. DON'T bugger the end of the jet with a poorly fitting screwdriver. Be VERY cautious about the idle jet, I have seen some jets that were really messed up at the screwdriver slot.

I always clean the various parts with carburetor/choke cleaner spray. For a major overhaul, you COULD take everything apart and use a commercially-available dip tank, which means you will HAVE TO replace the throttle shaft O-ring, as these 'hot tanks and chemicals' will ruin the O-ring; of of course with the shaft and butterfly removed, you'd replace the O-ring anyway.

There are many types of spray cleaners available at your local autoparts store. Some are NOT very good.   Brake cleaners are especially NOT good at cleaning carburetors.   A good cleaner will instantly dissolve a fair portion of the brown stain deposits, sometimes a cotton swab will help on the outside of the carburetor body. I prefer the spray by Berryman, called B-12 Chemtool, and only one version....Carburetor Choke Cleaner. This is nasty stuff. Use outdoors, or with your garage door open!   Spray, wait 15 minutes, spray again.  This works well on the tiny idle passageways, etc. If the Berryman product has a C or CA in the model number on the can, it is not as powerful as that shipped to other states.

Note:  The carburetor overhaul kit contains numerous rubber O-rings.  It can be confusing to know where they fit, as some are nearly the same size.  The author's website has the information.

Be sure the main jets and needle jets have the proper and same size number on them, left and right. If you have removed the idle pilot jet, be sure the numbers match.

Every once in awhile, someone asks about changing the setting of the slide needles. I point out that changing from the stock setting to one that is one step richer or leaner, is usually not necessary, as the problem is typically with float level, etc. However, even when there is a definite reason, such as EXTENSIVE modifications to the exhaust system, to move the needle position, please NOTE!... SOMETIMES 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. There ARE instances in which changing the needle position is quite appropriate, because the carburetor, even with proper float setting, is running, perhaps, lean.  Advice on this can be had on the Airheads LIST.   Generally speaking, ONE NOTCH change on the needle position is equivalent to THREE sizes of change on the associated jet!

***When installing or removing a slide needle, the proper method is to clean them and your fingers, so 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. UNDER NO CIRCUMSTANCES BEND THE NEEDLE. Rotate one direction, then the other, as required. Some may find that a thin piece of leather will help. NEVER use pliers!!! ONE (1) 90 degree rotation per so- called needle position!

NOTE!....some models of carburetors have the needle installed/locked using a small screw.  Some later carburetors have aluminum alloy needles.  The grooves on them wear rather fast, and inspecting the needle, and/or moving it up and down, tells the story.

***I recommend that you check and write down your slides/needles distance, to be sure they are the same, BEFORE removing or installing or changing the needle position. You cannot 'see' the needle INternal position, it is done by feel, and having a measurement (you need to be accurate to maybe .015") may save you some hassles. I measure them with a common vernier caliper. I suggest your measurement is from the underside of slide-to-tip distance, or, the distance from top of slide assembly 'tube' to needle tip.

Needle position (there are 4 positions available) is measured from the top slot position (#1) of the needle. Most needles are in the #2 or #3 position from the top.

Carburetors should be mounted squarely to the motorcycle. View from the top, and also from a few feet to the rear of the bike, they should look properly vertical.

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 joint interface, caused by some tiny gasoline weepage. I was never bothered by this, as the 'problem' is sporadic and minuscule, and there is no performance problem. 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, 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. 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.

Some carburetors have had leaky steel plugs on the domes...that shiny center area. NOT all carburetors have that style of top. You can easily test the plugs for leaks when the domes are off for servicing the carburetors.  Those dome plugs must NOT be loose nor leaking.   DO NOT allow any leaks...it will act like a torn diaphragm. They can be crimped or epoxied. Another test is that spray brake-cleaner testing at idle. After sealing a dome plug, some epoxy very small BMW emblems to the top dome plug.  My website  http://bmwmotorcycletech.info  contains an article with a very large number of Roundel emblems listed, with parts numbers and sizes....from BMW's usage on bikes...and cars.

The floats (fuel level in the bowl) are adjusted by bending the tab that the float needle and wire clip attach to. I have found a very small screwdriver does this OK, and works safer and better than unwieldy long nose pliers. Do one carburetor at a time. BE GENTLE AND CAREFUL!! Turn gas off. Remove that one carburetor's float bowl. Turn gas on, lift float VERY GENTLY (AND VERY SLOWLY as you approach shutoff) with a fingertip, until the gas flow JUST stops. At THAT POINT where the gasoline JUST stops flowing, the top of the floats are to be parallel to the lower body of the carburetor. DO NOT use ANY higher pressure with your finger in trying to find this proper point, as you could lift the float too high. I allow as much as .020 inch below, maximum. I have done some fine-tuning by playing with the float level, I suggest you do NOT. Be gentle and careful about doing any bending. Float level affects richness-leanness, and gas mileage.  Here is a HINT on making the float adjustment that is MORE ACCURATE: Raise the float so that the gas just shuts off, then lower the float a teeny amount until the gas JUST starts showing, and use that point for the parallelness.  That avoids the potential problem of lifting the tip of the sprung float needle.

Some literature shows float level being set with carburetor upside down...or sideways....and mechanically eye-balling it. That is obsolete.

You could, if you wanted to, measure the actual fuel level in the float bowls for setting floats, but it varies with the size (32 or 40 mm) of carburetor, and very early /5 carbs were also different. I think it best not to do it, so have not put the information on it here.

More detailed information on carburetors will be found in several articles on the author's website: http://bmwmotorcycletech.info

Revisions:
05/21/2017:  Minor clarifications