13 BMW 1977 R100RS Rebuild & Refinish Bing Type 94/40 Carburetors

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

Left Carburetor Model Mark

Right 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"

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.

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

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 Throat

Right Carburetor Inlet

Right Float Bowl and Float Assembly

Right Carburetor Float Bowl and Float Assembly Showing Main Jet in Middle and Idle Jet Below

Right Main Jet (170)

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

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 – Two O-rings

Right Idle Fuel Jet (45)

Right Idle Fuel Jet (45)

Right Carburetor Jet Passages Look Clean

Right Carburetor Jet Passages Look Clean with no Thread Damage

Right Carburetor Needle Jet (2.66)

Right Carburetor Needle Jet-Size is Hard to Read (2.66)

Right Carburetor Top Uses 2 Screws

Right Carburetor Top On This Model Uses  2 Slot Head Screws

Right Carburetor Top and Slide Spring

Right Carburetor Top and Slide Spring

Right Slide with Vacuum Holes and Jet Needle (and Smiley Face :-) )

Right Slide with Vacuum Holes to Underside of Diaphragm and Jet Needle [and Smiley Face 🙂 ]

Right Slide Diaphragm

Right Slide Diaphragm Has No Tears-Note Locating Tab on Edge

Right Idle Mixture Screw and Spring

Right Idle Mixture Screw and Spring-Screw Tip is Not Damaged

Right Carburetor Choke Housing and Throttle Assembly

Right Carburetor Choke Housing & Throttle Assembly-Note Dimple on Right Side of Choke Shaft

Right Carburetor Choke Disk ("R" on Shaft)

Right Carburetor Choke Disk-Note “R” on Shaft

Right Carburetor Choke Housing with Disk Removed

Right Carburetor Choke Housing with Cover & Disk Removed

Right Carburetor Choke Assembly Parts

Right Carburetor Choke Assembly Parts-A Bit Dirty

Right Carburetor Throttle Linkage Assembly

Right Carburetor Throttle Linkage Assembly Uses Two Plates-Inner One Has a Bend

Right Carburetor Throttle Spring Detail

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 Throttle Linkage Detail

Right Carburetor Butterfly Plate - Punch Mark at Top on Screw Side of Shaft

Right Carburetor Butterfly Plate – Punch Mark at Top of Plate Facing the Screw Head Side of Plate

Right Carburetor Butterfly Shaft Screws Peened

Right Carburetor Butterfly Shaft Screws Peened To Prevent Backing Out

Dremel Grinding Wheel

Dremel Grinding Wheel

Right Carburetor Removing Peened Threads of Throttle Shaft Screwd

Right Carburetor Removing Peened Threads of Throttle Shaft Screws Before Removing

Right Carburetor Throttle Shaft Screw Ground Down

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

Right Carburetor Heating Throttle Shaft Before Removing Butterfly Plate Screws

Right Carburetor Throttle Shaft with Countersunk Holes

Right Carburetor Throttle Shaft Side with Countersunk Holes

Right Carburetor Throttle Shaft Peened Screw Side

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 Shaft Orientation

Left Carburetor Choke Disk and Shaft with "L"

Left Carburetor Choke Disk and Shaft with “L”

Left Choke Housing and Disk are Grungy

Left Choke Housing and Disk are Grungy

Left Carburetor Main Jet (170)

Left Carburetor Main Jet (170)

Left Carburetor Needle Jet (2.66)

Left Carburetor Needle Jet (2.66)

Left Carburetor Idle Jet with Hardened O-ring

Left Carburetor Idle Jet with Hardened O-ring

Left Carburetor Idle Jet (45)

Left Carburetor Idle Jet (45)

Left Carburetor Float Pin-Serrated End

Left Carburetor Float Pin-Serrated End

Left Carburetor Float Pin-Tapered 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

Left Carburetor Throat Not Too Grungy

Right Carburetor Choke Assembly Parts

Right Carburetor Choke Assembly Needs Cleaning

After cleaning and polishing, they look like this.

Refinished Carburetor Parts

Refinished Carburetor Parts

Bing Badge - No White Lettering

Bing Badge – No White Lettering

Refinished Carburetor

Refinished Carburetor Parts

Refinished Carburetor

Refinished Carburetor Body

Refinished Carburetor

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

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 Outside

Refinished Inside

Refinished Inside

Refinished Back

Refinished Back

Refinished Front

Refinished Front

Refinished Top with Added Rondel

Refinished Top with Added Rondel

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