Airhead Tech Pages


Bing Carburetor Rebuild

J.R. Buchanan - jbuchana@buchanan1.net
OK people, we're here to talk about rebuilding Bing Constant-depression carburetors. While this article does not cover slide-type Bings, or the Dellorto's used on the R90S, the basics are the same.

First, I'd like to thank some people who helped me rebuild my R75/5 carburetors and write this article:

Tim Balough    (Yes, Gator's brother)
Webb Bernhardt
Bill Confer
Jon Diaz
Cindy Fort
Carl Kulow
Jay Moody
Ben Zaborowsky

If I forgot anyone, I apologize.

The first thing you'll need is a rebuild kit. You can get these with or without diaphragms. Even when they are not torn, older diagrams can stiffen up and limit the motion of the vacuum-operated slider. This is not good. If your bike is on the older side (and it likely is, or you wouldn't be considering a carburetor rebuild, would you?), you should probably consider new diaphragms.

When I (with a lot of help!) rebuilt the carburetors on my R75/5 I also had some rusted screws holding the tops on the carburetors, so I got some new, genuine Bing stainless-steel screws for the carburetor tops. They are really nice looking with their hex-heads, but then again, paying Bing prices, they'd better be!

Speaking of these screws, the old ones can be very tight. From tinkering with the bike in the past, I knew that, on my bike, at least a few of them were stuck. I didn't want this to interfere with the work on the day of the carburetor rebuild, so I decided to remove them about a week before.

Of the eight screws, five came out with no problem. On the others, I got to enjoy myself a little. First I got to try a trick that I'd learned from Jay Moody, a local BMW mechanic.

The point is to get more friction between the screwdriver tip and the screw head. To do this, dip the tip of the screwdriver in some valve-grinding compound. The grit "bites" into the metal when you try to remove the screw.

First try, beauty. The screw came right out!

Next screw, not so good. No matter how hard I pushed down, the tip wouldn't stay in the slot. Bummer. I tried a little mechanical kludge at this point. For the screwdriver, I substituted a screwdriver bit, the type that's used in those multi-tip magnetic screwdrivers. The ones that have the 1/4 inch hex body. I used a three inch C-clamp to hold this into the slot on the screw. The jaws of the C-clamp fit around the carburetor body perfectly. For good measure, I used the valve-grinding compound as well. This puppy was not going to come out! After building up this weird assembly, I turned the bit with a 1/4 inch ignition wrench. Success! The screw came out. Only one left to go...

I must have forgotten to knock on fake wood, thereby irritating the vinyl dryads. Bad mistake. When I tried my kludge on the screw that stood between me and mechanical nirvana, the slot was twisted completely out, leaving me with a slotless head smiling up at me from a recess in the top of my carburetor.

Robert Pirsig ("Zen and the Art of Motorcycle Maintenance") called this situation a "Gumption Trap" And right he was, late at night when something like this happens, it stops you cold. There might be a straightforward solution, but you can't see it. Going on in this situation often leads to grief. His advice is to walk away. Look at it again tomorrow. Good advice, and that's what I did.

The next time I looked at it, it wasn't fun, but at least I wasn't mad. I simply drilled the head off the screw, tricky, but not too hard. Then I popped the cover off the carburetor and backed the stub of the screw out with a small pair of locking pliers. The biggest problem here is getting the drill centered. If you don't, you can mangle an expensive part. If you have any doubt, I would suggest that you take this sort of problem to a professional. At the very least, don't do it when you're mad!

I put the carburetor tops back on with the new hex-topped screws and rode the bike until the carburetor rebuild.

Now to the actual rebuild.

When we rebuilt my carburetors, I asked Tim about the procedure as a whole. His response, paraphrased due to my poor memory, "Pretty much, you take it apart, clean the parts, and put it back together with as many of the new parts as you can fit in".

That's pretty much what my service manual says too. They have a few handy tips that I'll include later, plus they skip around a bit, but it's fairly straightforward.

Speaking of service manuals, make sure you have one. As I write this, I'm looking at the exploded view of a Bing carburetor in my copy of the Haynes "Owners Workshop Manual". That's where I'm getting the fancier names for the parts from. This picture is invaluable when you have the carburetor apart.

The first step of the rebuild is to remove the carburetors. My first inclination, as well as the instructions in the service manual would lead to removing each carburetor as a unit. Tim suggested taking the tops off first, and let them dangle from their cables. A bit of a time-saver, and it worked fine.

First, remove the cables. To remove the throttle cable, slacken its adjustment and rotate the end of the cable in its lever until you can slide its end out of the throttle lever through the provided slot.

On the later models, the choke cable is removed in the same manner. Earlier choke cables are removed by slackening a nut on the end of the chock lever.

The tops are held on with four screws, which, as I mentioned above, can be very tight. Use care in removing them.

With the screws out (actually you could do this before you remove the screws), you can disconnect the throttle and choke cables from their levers.

With this cover off, you will see a rubber diaphragm. Remove it by pulling it straight up. The carburetor slide and the needle will come out with it. Inspect the diaphragm for tears. A torn diaphragm will lead to a rough-running engine that is really down on power. As mentioned before, if the diaphragm stiffens up with age, it can affect engine responsiveness and even limit top-end power somewhat.

(also check the Bing CV Carb Diaiphragm Replacement page)

An important point to remember is to keep the parts of each carburetor separate from those of it's brother. Some are "handed", to use the terminology in my service manual.

With the covers off, my manual suggests checking the needle which dangles from the vacuum-operated slider and the brass jet that it runs in for wear due to vibration. I've got over 80000 miles on my R75, and they showed no wear. It'd be a good idea to check though, as some people report problems with this.

Another point to check is the needle position. The fat end of this needle is held by a spring clip in the vacuum-operated slider. There are several notches in the top of the needle, allowing it to be raised or lowered. This affects the mixture at various airflow levels. Look in your service manual and find out which is correct for your bike. Then see where your needles are actually set. It turns out that mine were set to two different heights. One was correct, the other off by one notch. It's no wonder I never could get the engine to feel quite right!

My manual discusses the need to ensure the proper setting of the needles, but it doesn't actually tell one how to accomplish this. Upon examination of the vacuum piston assembly, I discovered that this is not one of those things that becomes clear upon examination. I was baffled, so I had Tim show me how to do this. It's fairly simple. You gently grab a hold of the fat part of the needle (where it comes out of the vacuum piston) with some needle-nosed pliers, turn it 90 degrees and pop it to the next notch, turn it 90 degrees, and then pop it to the next notch and so on. The rotation is needed since the notches in the shank of the needle are located 90 degrees apart from each other. The needle position is set by starting at the top notch, then counting the notches as you move the needle.

When doing this, be sure not to mar the portion of the needle that runs in the brass jet. We've already inspected it for wear, we sure don't want to add wear while working on it!

As you take the carburetors further apart, you should compare the jet sizes to the sizes specified in your manual. Over the years, someone may have changed them. They might not even be the same from side-to-side! The person who changed the jets may have done it for a good reason, but chances are the engine will run better with the factory-specified jets. If you ever feel the need to change the jets, start from the factory settings.

Now remove the float bowl. It is held on with a wire bale (clip) that you can release with your fingertips. The bowl will be filled with gasoline, so be careful. Before you dump the contents, take a look in the bottom. Water will look like the "globs" that float around in lava lamps. It may be clear and hard to see, or it may be brown and nasty. Mine was brown and nasty.

At this point the carburetors are still attached to the engine and air-cleaner, but they are missing the tops and the float bowls. Now it's time to remove them.

First, remove the fuel hoses. This might be a good time to examine the condition of these hoses, and if necessary, get ready to replace them upon reassembly of the bike. If they are cracked or lack the resilience to seal well, definitely replace them. I prefer to use the nice German hose with the braided outer cover. You know, the stuff that matches the hose that came on the bike originally.

This hose is often used without any clamps, and I've found that in the summer I can get away with that. However, come winter and cold temperatures, the rubber stiffens and loses its resiliency. At this point, the least disturbance leads to dangerous fuel leaks. Not to mention smelly shoes and socks. I suggest the use of hose clamps. They do detract from the appearance a bit, but I find the peace of mind to be well worth any cosmetic disadvantages.

While you are at it, check your fuel filters. You do have fuel filters don't you? I did, but they were old and some junk got through them. I put a new set on and examined the old ones. There was a lot of brown junk in them, so much that I examined my tank with a flashlight and a mirror. It looked fine. I'm glad that most of this stuff didn't make it to the carbs.

After the float bowl is removed, remove all of the clamps that hold the air intake tube (the big tube between the air cleaner and the carburetor) in place. Then remove the clamps that hold the carburetor to the cylinder head spigot. The carburetor may then be removed by wiggling/twisting it out of the hose and spigot. Set the carburetors on the bench for the rest of the work.

This would be a good time to look at the hose stub that connects the carburetor to the cylinder head. It may have started cracking or losing its resiliency, either way, you're looking at a possible vacuum leak and consequent carburetion problems.

After this, check the metal stub that screws into the cylinder head. It is threaded in and might well be loose. I understand that the tool kit has a tool used to tighten it, however I don't have the factory tool kit. A previous owner lost it. I made do with a metal strap and some locking pliers.

With the carburetors on the bench, start taking them apart. I'll stress again that you should keep the parts of each carburetor separate from those of the other. It wouldn't be going too far to do all the work on one, then do the other.

First remove the float assembly. The float pivots on a pin at the back of the carburetor body, drive (push gently) this pin out of the pedestals that support it. Use a small drift punch (without a hammer!) or some similar object. A small metal rod may be used. One side of the pin is fluted and serves to hold the pin in place in its stand. This is normally the side that faces in towards the engine. Apply pressure to the other side. Support the pedestals with your fingers or a piece of wood. It's not a likely occurrence, but if you do break one off, your looking at buying another carburetor body. Not inexpensive.

Lift the float out. Carefully turn over the carburetor and catch the float needle as it falls out. Don't lose it!

Now remove the main jet and its holder. See the picture in the manual if you're not sure what a given part is.

There should be a washer between the main jet and its holder. The main jet is threaded into the holder, the holder is threaded into the body of the carburetor.

Once you have the main jet holder out, you should be able to shake out the needle jet and its holder as well. They are held in by the main jet holder.

Here's where one of my carburetors got interesting. The needle jet holder was munged up on one side. Some previous rebuilder had been at it with something sharp. Was it sealing adequately? I felt it best to get a new one. We were lucky, Indy BMW was less than two miles away, they were open and had one in stock. Not bad...

Now unscrew the pilot jet. Notice that it is sealed with an O-ring instead of a washer.

At this point, take out the pilot mixture screw. I'd call this the idle mixture screw, but that's not what the manual calls it... Don't lose the washer and spring!

About the only part you haven't pulled apart yet is the "Auxiliary Carburetor". You see, Bing CV carburetors don't actually have a choke. When you operate the "Choke" lever, what you are actually doing is rotating a disc-valve in the auxiliary carburetor. This is a small carburetor within the main carburetor that is used to provide extra air and a rich mixture when starting and idling a cold engine. Clever.

First remove the lever. It is held on with a single nut. Then remove the auxiliary carburetor cover. It is held on with four screws. It will come out with the auxiliary carburetor slide, which is actually a disk that rotates when you operate the choke. Behind the disc, there is a spring and a small O-ring. This whole assembly seams to be rather prone to leaks, cleaning and sealing it is one of the main purposes of this exercise.

A caution when disassembling the auxiliary carburetors. It looks as if the disk valve can go in in two different ways. Making a drawing might not be a bad idea. My Haynes manual has a good picture of this assembly as well.

While you are taking your carburetor apart, I would advise that you not remove the idle speed screw. Just leave it where it is. This will make it easier to get the bike running again when you put it all back together. It will need to be set, of course, but it should be close enough to start the engine.

I would advise against removal of the throttle plate (called the "Butterfly Valve" in my service manual). Some people do this, but unless your throttle shaft bushings are worn, allowing air to leak in, I don't see the point in doing this. If they are worn, any repair is well beyond the scope of this article. You can check the wear by trying to move the shaft in a direction that is perpendicular to the long axis of the shaft. Compare this to a carburetor that is working well. I wouldn't worry much, this isn't a weak point of Bings as far as I can tell.

In any case, remember the screws that hold the throttle plate on are staked in place. That means that after assembly, the ends were smashed a bit to keep them in place. It's no fun when your engine ingests carburetor parts.

Ok, now that it's all apart, it's time to clean the parts. This is the first step where you get to play with toxic chemicals! I know you've all been waiting for that!

When I'm cleaning a carburetor, I tend to rely on Gumout brand carburetor cleaner and compressed air. I like the carburetor cleaner in both the spray can form and the bucket form that allows you to dip the parts and soak them for a while.

You can use other brands of carburetor cleaner of course, and some people have reported success using mineral spirits (paint thinner used with oil-based paints).

The compressed air is handy for blowing out passages and blowing the solvent off the cleaned parts. You can live without it though. Just use an aerosol can of carburetor cleaner to blow out the openings.

A toothbrush is really handy for cleaning the castings off. Once I had a toothbrush start to dissolve from the cleaners, so pay attention.

Two warnings here: One is for your health, the other is for your carburetors health.

Be careful with the carburetor cleaner. Exposure to strong solvents can cause health problems. Read the label before you start working.

Now for your carburetors health. Don't poke out the jets with wires. It is very easy to change the flow characteristics of the jets. Clean them out with carburetor cleaner and compressed air.

Another warning: If you left the carburetor tops on the cables while you worked, you'll be cleaning them while they dangle from the bike. Be careful with that carburetor cleaner, some kinds can damage paint!

While your carburetors are apart, check the floats out. There are two possible problems here.

The float might be disintegrating. This is fairly obvious, and requires replacement.

The floats may have absorbed gasoline over the years and started to sink. This results in a rich mixture, possible hard starting and bad gas mileage. There are only two real ways to check this. You can weigh the float on a tiny scale and compare it to the figures in the service manual. You can float it in some gasoline and compare it to a good float. Neither method is too convenient is it?

Once all of the parts are clean, it's time for reassembly. You've read the cliche, "Assembly is the reverse of disassembly", right? Well, it's pretty much true in this case. Just use all of the new parts that will fit and don't worry about having a few left over. The kits fit more than one model, and will have some parts for other models of carburetors.

When you put the floats back in, check the level at which they close the needle valve. With the carburetor upside-down on the bench, slowly lower the float until the tab touches the needle. At this point, the float should be parallel with the carburetor body. If it is not, carefully bend the tab until it is. Notice that if you let go of the float, gravity will pull it further down. At this point it will no longer be parallel with the carburetor body. This is normal.

The carburetors go back on the same way they were removed. If you left the carburetor covers hanging on the cables, you'll put them back on after the carburetors are back on the bike.

When you put the diaphragm back, note that there is a tab molded into them which must be inserted into a slot in the carburetor body. This aligns the slider properly.

Did you check the needle position earlier? Do it now if you've forgotten.

After replacing the tops, but before tightening the hose clamps that hold the carburetors in, rotate the carburetors until they are level. A small carpenter's level may be used for this. Be sure the bike is on the center stand to do this.

The end of the throttle cable is then placed back into its opening in the end of the throttle shaft.

On late models, the choke cable is installed in the same way. On earlier models, the choke cable has no formed end, it's just a plain wire. Insert this into the clamp on the end of the choke lever.

With either cable, you must adjust the choke lever position. The point here is that with the choke full off, the lever on the carburetor should be all the way down. When you start moving the lever, both the choke levers should start moving at the same time.

On the later models, this is simple. There is an adjustment where the coke cable is attached to the lid of the carburetor. Simply adjust this until the above criteria are met.

On the older models, put the choke in the full off position. Move the levers on the carburetors as far down as they will go. Tighten the clamps on the levers, making sure that you don't bend the end of the cable,thereby throwing off the adjustment.

Now reconnect the fuel lines.

Check the fuel lines for leaks by turning on the petcock(s). If you have clear filters installed, you should see gas running through them, then it should stop after a while. Of course, none should leak onto the ground.

There are two possible places for leaks. One is from the hose connections. A first step in the quest to solve these is tightening the hose clamps, if you are using them. If this does not work, it is time for new hoses.

The other possible leak is at the float-operated needle valve. Sadly this is a weak point of Bings. They do this a lot. If you have this problem, gently tap the carburetor body. If the leak persists, you may have heavy floats or a bad needle valve. Replacement is the best solution in either case.

Now, before the engine is started, rough in the idle mixture screws. Gently, and I do mean gently tighten the screws until they stop turning. Using too much pressure at this stage can damage the needle and its seat. Now back the screws out by the number of turns in your service manual.

Now it's time to synchronize the carburetors. This is a useful skill at every tune-up, even if you never do rebuild the carburetors yourself.

To synchronize the carburetors, I use a set of dial-type vacuum gauges. Many people prefer mercury manometers, known as "Carb-Sticks" There are advantages and disadvantages to each method.

The biggest advantage of the mercury manometer is repeatability and precision. Since the readings on the manometer depend on the earths gravity field being the same for each tube and the specific gravity of the mercury in each tube being the same, you can trust the readings.

The disadvantage of the mercury manometer is the poisonous nature of the mercury that is used. Take this stuff seriously. Don't spill it. When you aren't using the manometers, plug both ends of the U-tubes. Mercury is a cumulative poison. It looks rather inert, but it does evaporate into the air you breath. At first you don't notice any effects, then twenty or so years later -nerve damage. Hatters used to use mercury when making hats. I'm sure you've heard the phrase "Mad as a Hatter". Mercury poisoning.

The advantage of the dial-type gauges is their lack of mercury.

The disadvantage of the dial-type gauges is that the readings on the gauges are only as good as the quality of the gauge, and they may drift out of calibration over the years.

You can test for this imbalance by hooking them all up to an aquarium manifold (all the best mechanics shop at pet-supply stores) and connecting them up to one carburetor. When the engine is dialing, they should all read the same. Many gauges have an adjustment screw to correct them if they do not.

I don't like mercury around the house, so I use the dial-type gauges. I've been using the same set for about five years and the bikes I set with them run fine. If I start having problems, then I'll worry about the gauges.

A tip if you use the dial-type gauges. Unlike the mercury in the manometers, the needles in the gauges are not naturally damped. They'll vibrate badly as the engine runs. The gauge set should come with some tiny valves. These go into he hoses that connect the gauges to the carburetors. Adjust the valves until they restrict the air flow enough to almost stop the vibration. If you close them all the way, the needles won't move as you make changes, so be careful.

The first step of the carburetor synchronization is to warm the engine up. It's important to get it up to full operating temperature before you adjust the carburetors. It's not enough to idle it a few minutes, no matter how hot the cylinder heads get. Skimp on the warm-up, and you'll be wanting to re-synch the carburetors by the end of your next day of riding. Go for a ride. Maybe ten minutes or so.

If the carburetors are too far out of synch to ride, set them roughly, using the following directions, then go for a ride and when you get back, set the carburetors carefully.

With the engine warmed up, connect the gauges or manometers. They connect to small nipples on engine side of the carburetor. Some older Bings do not have this nipple and were originally adjusted using another method which is covered in your service manual.

This method involves pulling one spark plug wire at a time. You can get an unpleasant shock this way. It's really no too good for the coils, but with the low energy levels of the old points-style ignition systems, no damage occurs to good parts. Electronic ignitions deal with enough energy that problems can occur if you deprive it of somewhere to go. Never pull a plug wire from a running engine that has electronic ignition. A shock is much more likely also. In fact some manuals warn that the shock from an electronic ignition can be deadly. I have no intention of testing that theory.

I'd suggest adding the nipples. Bing has a kit to do this, or you can look at a bike that is equipped with them and roll your own, assuming you have enough confidence. It's well worth the effort.

Slide-type carburetors use the pulled-plug-wire method of adjustment as well. Vacuum nipples may be added to these carburetors as well.

Check the throttle cables to make sure there is some slack. The point here is to make sure that the position of the throttle plate is determined by the idle speed screws. Until you rotate the twist-grip, the cables should be slack and have no effect on the throttle position. If you can pull the jacket of the throttle cable up by at least 1/16 inch, you should be in good shape.

If there is not enough slack, loosen the lock nuts that hold the cable ends and rotate the ends of the throttle cables in their threads in the carburetor tops until you have some slack. Tighten the lock nuts loosely, you'll need to loosen them again later.

Start the engine.

Adjust the pilot (idle mixture) screws for the highest reading on the gauges. Do one, then the other, repeating until you are satisfied. They will interact a little. Many people leave them at this point. I usually turn them in (leaner) by about 1/4 turn at this point. I like to think I get slightly better throttle response and a little better gas mileage. It's probably all in my head, but I'm happy.

Now look at the gauges/manometers. The readings should be the same, but at this stage they won't be. What you have to do is adjust the throttle stop screws until the readings are the same and the idle speed is correct for your model. These adjustments interact quite a bit, so it will take you a little while to get the hang of setting them.

Turning one of the idle screws clockwise raises the idle speed and lowers the vacuum reading on that side. Since the engine is now turning faster, it tries to pump more air through the other side as well. This raises the vacuum on the other side. It takes some practice to get the hang of this.

Once the idle speed is set, go back and do the idle mixture adjustment again. It probably won't need much, if any, adjustment.

Now check the idle speed and gauge/manometer readings again. They'll probably be fine, but tweak them if they are not.

Now it's time to adjust the throttle cable free play. This is adjusted by turning the threaded ends of the throttle cables where they pass through the carburetor tops.

First loosen the lock nuts that hold these ends tight.

Rotate the twist-grip until the engine is turning 2000 RPM. This is what the "Tune-up" screw on the earlier boxers is for. It holds the throttle open for this step. It make a handy "Cruise Control" as well. If you don't have one, you'll have to hold the throttle open with one hand.

Check the gauges/manometers. Once again, the readings should be the same, but once again, they won't be. Set them using the same basic procedure you used for the idle speed screws, but this time don't pay much attention to the engine speed. As long as it's around 2000 RPM, don't sweat it. This time you want to get the same gauge/manometer readings with a secondary goal of having just a little slack in the throttle cables at idle. I try for a little under 1/16 inch.

If you get too much slack, you'll have an awful time applying throttle smoothly in off-idle situations. If you don't have enough slack, you can wind up with the dreaded "Engine revs as the bars are moved" syndrome.

When you get the throttle cables set, don't forget to tighten the locknuts.

The throttle cable adjustment is the one that tends to drift fastest. Often when your bike starts to get that off-idle shudder, a quick adjustment of the throttle cables is all that is needed.

If your throttle cables are old and worn, this adjustment will be needed more frequently. Even with new cables, this is the most frequently needed carburetor adjustment.

A word of caution. If you spend too long making these adjustments, especially at 2000 RPM, you can overheat the engine. It's nothing to worry too much over, as it's not much worse than inching along in a traffic jam, but if the engine gets too hot, it will be hard to get the adjustments set correctly. A box fan aimed at the front of the engine is a real help, but it's best to try and do the job rapidly. If the engine gets too hot, remove the gauges and go for a ride to drop the engine back to normal operating temperature.

During this procedure, I've seen more than one boxer weep a little transmission fluid from the breather hole in the battery ground cable bolt. Just wipe it up and cool off the engine. No worry.

By the way, if your valves are needing adjustment, do them before the carburetors are synchronized. Changing the valve lash has a small but very real effect on the carburetor adjustments.


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Last Update: 11 September 2010