Contents
Bing Carburetor Models on R100RS Bikes
The Bing Carburetors on the bike are the constant velocity type, models 94/40/103 and 104 (odd number is left side, even number is right side) which are fitted to the CFO engines and are a different model from those used on the standard 1977 R100RS engine. The model number mark is on the outside face of the carburetor body on the reinforcing web between the top and throat.
Left Carburetor Model Mark
Right Carburetor Model Mark
The standard engine uses the model 107 & 108 carburetors which have the same size outlet, 40 mm, as the 103/104 models. Several different models of the 40 mm carburetor are used on the R100 bikes and I tried to identify the Bing model carburetors BMW used on the twin shock R100 bikes up to 1984 when they were discontinued. You can find that information here.
Parts List
The list of parts I used is below.
| Part # | Description | Qty |
| 13 11 1 336 902 | SET: GASKET SET CARBURETOR (from 09/78) |
1 |
| 13 11 1 335 321 | NOZZLE NEEDLE | 2 |
| 13 11 1 261 702 | JET – 2,66 (from 09/79) | 2 |
| 13 11 1 254 768 | PIN | 2 |
| 13 11 1 254 766 | FLOAT | 1 |
| 13 11 1 335 318 | FLOAT NEEDLE | 2 |
The needle jet (13 11 1 335 321) is not what’s listed for the 1977 R100RS as the listed part is for the standard motor and carburetors. I have the CFO version with 38 mm exhaust and it uses a different carburetor series (103-104) and the smaller jet needle.
One float (13 11 1 254 766) had been replaced so I opted to only replace the older one. Normally I would replace both but I believe the new one was recently installed.
How The Bing Constant Velocity Carburetor Works
The primary function of a carburetor is to mix fuel with air in the right proportions so it’s a combustible mixture when it’s in the cylinder and the spark plug “sparks”. To do this, it has to operate over a wide range of external conditions; air pressure, temperature and engine speed. The Bing constant velocity carburetor uses several “circuits” to help control the air fuel mixture over the range of operation.
- Constant Velocity Circuit
- Main + Needle Jet Circuit
- Enriching Circuit (aka, Choke)
- Idle Circuit
The design adjusts the metering of fuel to account for changes in air pressure due to altitude and temperature via the Constant Velocity circuit. The Main Jet Circuit provides a correct mixture from off-idle to full throttle. It is connected to the Constant Velocity circuit so the amount of air and fuel are adjusted for changes in altitude and temperature. For a cold engine and/or cold outside conditions the Enriching Circuit makes a richer (more fuel) mixture that burns at these conditions. The Idle Circuit provides a proper mixture when the engine is running at a low RPM and the main jet circuit can’t do so.
I rely primarily on the published information about the Bing carburetor functions provided in the “Bing Carburetors”, aka “The Bing Book” published by the Bing Agency International, LLC.
Bing Agency “Bing Carburetors”
Constant Velocity Circuit Operation
Here is a short video showing how the constant velocity circuit works.
Main + Needle Jet Circuit Operation
Here is a short video showing how these components work in the main+needle jet circuit.
Enriching Circuit Operation
Here is a short video showing how the enriching circuit works.
Idle Circuit Operation
Here is a short video showing how the idle circuit works.
Video: Rebuilding Bing CV Carburetors
I made a general video about rebuilding Bing Constant Velocity (CV) carburetors. The technique is applicable to 64/32 (32 mm) and 94/40 (40 mm) CV carburetors. There are some differences in certain models including the choke and throttle linkages and the spring added to the top of slides in some models. So, you will see all the specific details in this write-up.
VIDEO: Rebuild Bing 94/40, 64/32 CV Carburetors
Overview of a Carburetor Rebuild/Refurbish
In my experience, it’s worth the cost in parts to completely rebuild the carburetors: replace all o-rings and gaskets, replace the jet needle and needle jet, replace the float needles and replace the floats if they are discolored. It’s frustrating to debug problems caused by air leaks, worn needle jets and jet needles leaks from the float bowl, etc., so a complete carburetor rebuild provides a known baseline.
I soak the parts and carburetor lower body in Berryman’s B12 ChemTool. It’s very nasty stuff and you need to wear rubber gloves, eye protection and ventilate the area you are working in. It does a very good job of removing varnish, carbon and baked on fuel deposits. I cover the containers I soak the parts in to prevent evaporation and the collection of fumes. I separate the jets from the choke parts and soak them in small containers. I put the body into a margarine tub and fill it until the B12 level covers the bottom of the venturi so all the fuel passages are submerged. When I’m done (a couple hours is sufficient) I pour it back into the can, mark it “Used” and reuse it. It takes about 1 1/2 cans to fill my containers deep enough to clean the parts, YMMV.
Reusing ChemTool B12 Cleaner
Disassemble Carburetor
The procedure is the same as the 1984 R100RS Bing Carburetor rebuild I documented here.
I take pictures of the condition of the parts and the numbers on the main jet, needle jet and idle jet. The following pictures show what I found on the right carburetor and then the left. There were some surprises, but no signs of damage to the carburetors.
Right Carburetor Disassembly and Condition
Right Carburetor Inlet
Right Carburetor Float Bowl and Float Assembly Showing Main Jet in Middle and Idle Jet Below
Right Main Jet (170)
The right float bowl gasket got crimped. This can allow air leaks into the bowl and fuel leaks.
Right Float Bowl Gasket is Crimped
When I remove the right idle mixture jet, there are two o-rings on it: there should only be one. Both o-rings are hardened.
Right Idle Fuel Jet – Two O-rings
Right Idle Fuel Jet (45)
Right Carburetor Jet Passages Look Clean with no Thread Damage
Right Carburetor Needle Jet-Size is Hard to Read (2.66)
Right Carburetor Top On This Model Uses 2 Slot Head Screws
Right Carburetor Top and Slide Spring
Right Slide with Vacuum Holes to Underside of Diaphragm and Jet Needle [and Smiley Face 🙂 ]
Right Slide Diaphragm Has No Tears-Note Locating Tab on Edge
Right Idle Mixture Screw and Spring-Screw Tip is Not Damaged
Right Carburetor Choke Housing & Throttle Assembly-Note Dimple on Right Side of Choke Shaft
Right Carburetor Choke Disk-Note “R” on Shaft
Right Carburetor Choke Housing with Cover & Disk Removed
Right Carburetor Choke Assembly Parts-A Bit Dirty
Right Carburetor Throttle Linkage Assembly Uses Two Plates-Inner One Has a Bend
Right Carburetor Throttle Spring Detail With Spring Detached From Hole in Bottom of Bracket
It’s easier to remove the spring from the edge that the shortest distance from the end of the slot.
Right Carburetor Throttle Linkage Detail
Right Carburetor Butterfly Plate – Punch Mark at Top of Plate Facing the Screw Head Side of Plate
Right Carburetor Butterfly Shaft Screws Peened To Prevent Backing Out
Dremel Grinding Wheel
Right Carburetor Removing Peened Threads of Throttle Shaft Screws Before Removing
Right Carburetor Throttle Shaft Screw Ground Down
If someone had removed the throttle butterfly previously, it’s common to use blue Locktite to secure the screws even if they have been peened. I use a heat gun to heat the throttle shaft so any Locktite will melt before trying to remove the screws. They came loose easily.
Right Carburetor Heating Throttle Shaft Before Removing Butterfly Plate Screws
Right Carburetor Throttle Shaft Side with Countersunk Holes
Right Carburetor Throttle Shaft Side for Peened Screws
Left Carburetor Disassembly and Condition
And from the left carburetor.
Left Carburetor Choke Shaft Orientation
Left Carburetor Choke Disk and Shaft with “L”
Left Choke Housing and Disk are Grungy
Left Carburetor Main Jet (170)
Left Carburetor Needle Jet (2.66)
Left Carburetor Idle Jet with Hardened O-ring
Left Carburetor Idle Jet (45)
Left Carburetor Float Pin-Serrated End
Left Carburetor Float Pin-Tapered End
Clean and Refinish
I show how I do this work here:
These carburetors are not real grungy.
Left Carburetor Throat Not Too Grungy
Right Carburetor Choke Assembly Needs Cleaning
After cleaning and polishing, they look like this.
Refinished Carburetor Parts
Bing Badge – No White Lettering
Refinished Carburetor Parts
Refinished Carburetor Body
Refinished Carburetor Top
Assemble Carburetors
The procedure for the 1977 R100RS caburetors is the same as the 1984 R100RS I documented here.
The Bing Book contains the needle jet settings for the various carburetors used on BMW airheads. The 94/40/103-104 models have the jet needle in the 3rd position from the top. It can be hard to know for sure which position the needle is in: sometimes the needle jumps over a position. I decide to measure the distance from the tip of the needle to the bottom of the slide for each of the four jet needle positions.
Measuring Jet Needle Height
The table below shows what I measure for the needle position.
| Needle Jet Positions | |
| Position | Distance (mm) |
| 1st | 48.9 |
| 2nd | 47.2 |
| 3rd | 45.5 |
| 4th | 43.8 |
Final Result
Here is one of the carburetors after being rebuilt/refinished. I decided to add a BMW Rondel to the top of the carburetor dome to see if I like this added bit of bling.
Refinished Outside
Refinished Inside
Refinished Back
Refinished Front
Refinished Top with Added Rondel
Revisions
2023-12-22 Added Video section about rebuilding Bing CV carburetors.
great job describing the process. Thank you very much
Hi Herman,
Thanks. It’s good to know this write up was helpful to you.
Best.
Brook.
This is amazing!!! All the information I need in one place. According to my spark plugs my right carb is running rich. Im planning on pulling the right carb apart, anything I should keep an eye out for? Many thanks
Hi Peter,
I tried to put all the details together from various sources. It sounds like that was what you are looking for. Cool beans.
In answer to your question, I’d check/do the following:
1. Verify your valve clearances are correct on the right side.
2. Is right side enricher (aka, choke) closed all the way?
3. Verify that the gasket on the choke housing is not cracked, sucked into the cavity or otherwise not sealing.
4. Verify that the jet needle is in the correct position. I’ve seen folks get the needle clip position on one carb correct and the other off a slot.
5. Verify the float height on the right is correct. A difference in fuel level in the float bowl changes the mixture.
6. Verify the float is not “fuel logged” making it heavier so it doesn’t shut off the fuel flow soon enough causing high fuel level in the bowl. Sometimes folks have this problem, and try to adjust the float level to compensate, but eventually the float won’t close the float needle and then the carb leaks (not your problem at this point).
7. The needle jet and jet needle wear due to vibration. That increases the mid-range richness. When I rebuild, I always replace these. That said, both sides should wear the same, so not as likely the cause of your problem.
8. Verify that the main jet sizes are the same.
That’s about all that comes to mind. It sounds like you plan to rebuild the carb(s) with new o-rings and gaskets. If so, depending on the age of the carbs, I’d consider replacing the floats and while I’m at it, the float needles to eliminate any problems from these.
I hope this helps.
Best.
Brook.
Great material, very educational, exactly what I needed to go into action. Before getting started, I still have a possibly naive question: How to you remove the black plastic plate very visible on the picture captioned “Bing badge no white lettering” ? It looks to me attached by sort of rivets…
Yves,
I’ve not removed these before, but I believe they are secured by a rivet. You can contact the Bing Agency for details about these badges and the way they are mounted.
–> https://bingcarburetor.com/index.html
I hope that helps.
Best.
Brook Reams.
Thanks. In fact I removed one carb today and had a quick look. I have not received yet the set of replacement seals, but needed a sense of the difficulty. I believe now it is not needed to remove the plastic black Bing plate, the pivot can probably by removed from the other (inner, closest to engine) side. My goal is primarily to change the O rings. The other piece of good news is that on my bike, it does not seem to be required to grind the tips of the screws attaching the circular flap to the pivoting lever. I was a bit nervous about grinding and abrasive dust filling the carb.
I spent a lot of time looking at your pictures, amazed by your outstanding skills to transform a piece of junk into a brand new magnificent machine… Congrats !!!
Yves,
Best of success in your rebuild. I’m pleased my documents have been helpful.
Best.
Brook.
Hi Brook, thanks for great educational material for these bikes, the best I have seen! I have a 1977 R100RS for over 30 years, I have rebuilt or replaced about everything on it, with many upgrades .(I wished I had access to this information back then…before the internet). My best friend Rusty owned a 1984 R100RS last edition, he died 2013 and wanted me to have his bike. I have been doing working on it this winter. Now ready to go to the carbs. Few questions:
I live in Boulder, think your in the Denver area? 1) Did you change the jetting for your R100RS for the altitude? If so what was changed? 2) How many miles before the 40mm bings need rebuild? (ie., needle jets/needle, Diaphragms, etc.? the ’84 has about 60K sure its time).On my 1977 R100RS I have noted I better thottle responce with new diaphragms, even when no holes, maybe they lose elasticity with age and exposure to fuel? Whats your thoughts?
Let’s meet up sometime for tire kicking and riding.
Charles
Hi Charles,
I sent you an Email. Thanks for stopping by and looking at the documentation.
Best.
Brook.
recently acuired a 84 r 100 that has some carb probs. at one point when running a idle fuel was spurting out from under the plastic bing badge. This seems to be a prob. if badge does not come off easily
Hi Cecil,
That is a symptom of too high a fuel level in the float bowl. You don’t need to remove the Bing badge to fix the problem. There is an air hole behind the badge that lets fuel exit when the fuel level is too high in the float bowl.
Causes of fuel overflow from the carburetor include:
– float set too high so fuel level is too high
– dirt, grit, tank debris stuck in the float needle passage preventing the needle from seating
– Float needle tip (it’s rubber IIRC) is worn letting fuel leak
– wrong style float needle installed (early steel tip needle in seat designed for rubber tip needle)
– worn float needle seat prevents float needle from sealing
– float is old, and “fuel logged” so it sinks into the fuel
– float arm is stuck on the pivot pin and doesn’t let the float ride up all the way.
My usual approach to a newly acquired airhead is to do a complete carburetor rebuild as soon as possible. That usually removes a lot of pesky running problems that will plague you.
Hope that helps.
Best.
Brook.
Hello, super job – I now bought R75/5 year71. I have question – Carburator the same on this photo BING, how to set the same course of carburetor cables? The vacuum gauge cannot be connected anywhere.
I seen on video, on this carburator have screw, but i dont have screw there. Thank you
Hi Petr,
The early /5 bikes, like yours, used a Bing model that did not have the vacuum port and screw. I believe your Bing carburetors will show “64/32/3” and “64/32/4” on the name plate on each of the carburetors. My bike is a 1973 with “64/32/9” and “64/32/10” models that do have the vacuum port.
I hope that helps.
Best.
Brook.
I have a 1979 bmw r100rs. I have stripped it back to bobber styling and have a GP style short exhaust, and k+n style filters. When the bike is up to temperature and switched off and left for 5 mins it turns over but won’t fire. It’s fine running, starts well from cold and starts well immediately after being switched off. I’m convinced it’s carberation, and know motocross bike have a hot start lever which allows more air and weakens the mixture. I’m thinking it needs something like this but aware no one else seems to have this problem. Any ideas?
Hi Chris,
Well, you changed just about everything that affects the jetting (idle, mid-range, full throttle) on these carburetors. The airbox and the back pressure from the exhaust all influence the stock Bing jetting: The airbox design is particularly influential from what I have heard.
I have no experience making the modifications you made, so I have no advice on how to proceed. I would call The Bing Agency in Kansas (https://bingcarburetor.com/index.html) and discuss with them the changes you made to exhaust system and K&N pods and ask them for advice on what jetting to use.
I hope that helps.
Best.
Brook.
Ok thank you. Have you ever come across the scenario I described in terms of not starting when hot due to carberation?
Chris,
No. But I have seen that problem with ignition system failure when hot.
1. Coils are cracked
2. Electronic ignition Hall effect sensor fails when hot. (IIRC, your bike, if it is the stock ignition system, does not have those.)
There is an old saying: “90% of carburetion problems are ignition problems, and 90% of ignition problems are carburetion problems.” I pass that along somewhat in jest, but also to suggest that when your mind gets fixed on what you think the problem is, it stops looking for other potential causes. It’s just human nature.
Carburetion problems are a question of air/fuel ratio, or what we call running rich or lean. You can use the spark plugs to tell you about how rich/lean the carburetion is. For example, run the bike at 4000 RPM, hit the kill switch, pull over, pull the plugs and look at the insulators. White=lean, Black=rich; you want light tan for correct air/fuel ratio. This test can be done at idle, mid-range and full throttle so you can understand how all three carburetor circuits are metering the air/fuel ratio.
Since the choke (aka, enricher since the BING carburetor does not restrict air to make the starting mixture richer) will richen the mixture, you could put on a bit of choke when it’s hot and see if it will start. If so, the idle mixture is too lean.
You mention motocross bike carburetion. Those are slide carburetors and how they work does not apply to BING Constant Velocity (CV) carburetors. I posted several videos on my YouTube channel that explain how the BING carburetor works. If you are going to customize your engine as you did, understanding the theory of operation of the BING carburetors is essential to getting them working properly and the diagnosing any problems.
–> How Bing CV Carburetors Work
And, as I said before, talk with the Bing Agency. They can advise you of jetting that will get you close to where it ought to be. From there you can adjust in small steps to get the optimum carburetion.
I hope that helps.
Best.
Brook.
Thanks very much. I’m going to go through everything that might make it run hot now inc spark plug types and ignition timing. The exhausts glow red on tickover, the plugs are biscuit brown so look ok. When it doesn’t fire hot, once it’s cooled a little it will fire on tickover but cuts out if you open the throttle too quickly. I’ll baseline everything and go from there. I’ve had the cracked coil issue where it heats and swells and stops sparking but I’ve pulled a plug when it won’t fire and there was still a good spark. I’ll keep at it.. thanks
Chris
Excellent write-ups, you probably don’t realize how many of us will continue to use your detailed pics and write-up to keep out beemers running silky smooth. Thanks again! Roy
Hi Roy,
You’re welcome. The more of these cool machines we can keep out of the scrap pile or parted out, the better.
Best.
Brook.
Thank you for this site.I learned much. I have had 6180418 about 40yrs. George Carl
Hi brook ,1977 BMW R100S,my Bing vacuum takeoff screws are stuck and i do not want to apply to much pressure is there a hole i can pour an anti-seize down,or what would you suggest? .Regards Ian.
Ian,
I’d try some heat from a heat gun around the carb where the screw goes in. The carb is made mostly of Zinc which should expand more than the steel screw. I would also pull the carb off the bike, and apply some anti-seize around the screw and let it sit. That may help get the threads lubricated.
I hope this helps.
Best.
Brook.
Ok thanks for the tips Brook,will have a go. Regards Ian.