Introduction

I’ve posted very detailed carburetor and Karcoma petcock rebuild / restore procedures for a 1973 R75/5 that uses the Bing 64/32/10 (right side) and 64/32/9 (left side) carburetors here:

• 13 BMW R75/5 Rebuild Bing Carburetors & Karcoma Petcocks

This is a 32 mm inlet constant velocity carburetor design.  The 1983 R100RS has 94/40/114 (right) and 94/40/113 (left) carburetors which are larger types with the same constant velocity design.

The type numbers are stamped into a rib near the top of the carburetor.These are 40 mm inlet carburetors.

On the other side of the same rib is the date of manufacture.

Resources

Here is a link to the theory of operation of the Bing type 94 carburetor:

• Bing Constant Depression Carburetor Type 94

Bob Fleischer has a lot of information about various types of Bing carburetors on his web site:

• Bing CV Carburetors and Overhaul-Part 1 of 2 Parts
• Bing CV Carburetors, Part 2-Notes, Tuning … and Overhaul Items

The exploded view diagrams for these two types are the same as shown below, even though some of the parts are different on the two types.

Video: Rebuild 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

Some Differences Between Type 94 and Type 64 Carburetors

For the R100RS type 94 Bing carburetors, differences include the top cover (20) with only 2 screws (21), internal spring on the slide (22),and the choke return spring (52b).

Also, the throttle plate adjustment is difference on the type 94 and DOES NOT have screw (36), spring (37), and bracket (31), instead, using screw (34) and the longer bracket (31) with the tabs (27a/b) and (28) with spring (35). Of course, the internal jets are different sizes, but they go in the same place as the jets for the Bing type 64 carburetors.

Differences in 1973 R75/5 vs. 1983 R100RS Karcoma Petcocks

There are a couple differences in the Karcoma petcocks. The  R100RS has an outlet 90 degrees from the inlet while the R75/5 outlet is directly below the inlet.

The internal filter screens are different; the R100RS uses a much longer screen and has the gasket built into it while the R75/5 screen is shorter and uses a separate gasket.

Parts List

The following is a list of parts, part numbers and quantities. I had to replace the choke and throttle return springs as these were pretty corroded. I also decided to replace the throttle and choke cables (not shown in the parts list) to avoid surprises that come from unseen corrosion and bent or broken cable strands.

Part #	Description	Qty
16 12 1 234 869	Karcoma, O-ring	2
16 12 1 238 924	Karcoma, Rubber gasket, 5-hole	2
16 12 1 240 060	Karcoma, Fiber gasket, Top	2
16 12 1 233 367	Karcoma, cap petcock	2
13 11 1 336 902	40 mm Carb Rebuild Kit, Pair	1
13 11 1 254 766	Float	2
13 11 1 254 768	Float Hinge Pin	2
13 11 1 335 318	Float Needle	2
13 11 1 335 320	Needle Clip	2
13 11 1 261 702	Needle Jet 2.66 (From 09/1979)	2
13 11 1 335 321	Slide Needle 40mm Carb	2
13 11 1 254 738 SS	Throttle Shaft Screw SS	4
13 11 1 259 870	Vacuum port gasket	2
13 11 1 258 319	Fuel Line, Braided	4
13 11 1 335 312	Throttle Return Spring, 40 mm	2
13 11 1 337 409	Choke Return Spring	2
13 11 1 337 361 SS	Dome Top Screw	4
13 72 1 264 392	Head to Carb Rubber Sleeve, 40 mm	2

Cleaning and Polishing

To restore the finish on the carburetors takes a lot of elbow grease and patience, but the result is worth the time.  This section from my rebuild of my 1973 R75/5 carburetors provides the details of what I have found that works.

• R75/5 Cleaning and Polishing Link

Here are some of the before pictures from the R100RS type 94 carburetors.

Rebuild Karcoma Petcocks

Since the rebuild procedure is quite similar between two versions of the Karcoma petcocks used on the R75/5 and on the R100RS, I’m going to reference the write-up I did for the R75/5 and note any differences I found when rebuilding and refinishing the straight and 90 degree outlet Karcoma petcocks.

Here is the write-up for the straight version of the petcock.

• R75/5 Rebuild Karcoma Petcocks Link

First, I drained the gas from tank. I used an upended trash basket to hold the tank above a 5 gallon gas can.

When I unscrewed the petcock from the tank, there was quite a bit of junk in the bottom of the tank that had collected on the strainer screens. The screens were in good condition and doing their job so I did not replace them.

This is the order of the fuel tubes. The short one is for reserve and goes in the hole closest to the outlet while the longer one is for the main.

I use a Philips head screw driver to remove the plastic cap from the petcock as it reduces the chance of damaging the plastic or your hand.

You can see the serrations on the nut that holds the petcock handle and internal parts together. The black plastic cap is secured by the serrations.

When I get one side of the plastic cap free of the metal cap, it’s easy to remove the black plastic cap. Then I slide it over the petcock handle to remove it. It just barely slides over the wide part of the handle.

I use a wide,smooth jaw (without serrations on the jaws) pair of pliers to grip the serrated ring and turn it to remove it from the body of the petcock. You may need to use some Kroil or other penetrating oil if you can’t get the ring to budge. Try not to flatten the serrations as these help hold the black plastic cap on.

Once the serrated cap is off, the inner spring and parts can be removed. I remove the serrated cap over the handle just as I did the black plastic cap.

I remove the black rubber 5-hole gasket from the petcock body using a pick and I use the pick to remove the o-ring from the handle. I replace them with a new 5-hole gasket and o-ring. I put a bit of silicone grease on the o-ring to help lubricate it.

I clean and polish the body of the petcock and then reassemble it. Note, the internal plate on the handle has a tab that fits in a slot in the petcock body to align the handle.

Getting the serrated ring to thread into the petcock body is hard due to the strong spring you have to push on while twisting the ring. I align the start of the threads on the ring with the start of the threads in the petcock body so when I twist the ring thread it will immediately engage the thread in the petcock body. I put the petcock body on the floor and then put my weight on my arms to force my fingers down on to the ring and then twist my wrists a quarter turn to engage the ring in the threads. After a couple tries, I get the serrated ring threaded into the petcock body and then use the smooth jaw pliers to tighten the ring. Be careful you don’t cross thread the ring in the body.

I install a new black plastic cover on the petcock by slipping down the handle, aligning it with the petcock and pressing it over the serrations. Then I install the plastic fuel pipes into the body of the petcock and check they are in the correct hole by rotating the petcock handle to “on” and blowing through the long tube to be sure air comes out the outlet. If it doesn’t, I have the tubes reversed.

The petcock nut has a reverse thread on one side. I just start the nut on the petcock body and then turn it on the tank threads while holding the body until it the nut is tight. Then I use a crescent wrench to tighten the nut.

Carburetor Disassembly

The procedure is I use for 94 model carburetors is the same as I show for the R75/5 type 64 carburetors. You can see how I did that work here.

• R75/5 Carburetor Disassembly Link

Due to rusty top cover screws, I soaked them in Kroil for a couple days and then used my impact driver to loosen them. I don’t want to break off the screws and have to try to use an easy-out to remove them.

DANGER:
I learned after using the impact driver the way I show below that I was in danger of breaking the top edge of the carburetor casting. It should be supported by putting the bottom edge of the casting on the edge of the work bench. That way when you hit the impact driver you shouldn’t break the edge of the casting. I was lucky and didn’t need to hit the driver very hard to break the screws loose.

When I removed the left air tube, a plastic tube was loose inside.

Further inspection of the air box showed this was one of the two oil breather pipes that direct crankcase vapors back into the carburetors. The type 64 used on the R75/5 has a single large black plastic pipe that only goes to the right side air tube. The air box on the 1983 R100RS was redesigned to aid meeting air pollution requirements. The oil breather system is different. The rubber tubing that connects to the plastic pipes inside the air tubes was split, so I ordered new rubber tubes and clips to secure them.

The air tubes have a metal ring that fits inside the rubber gasket and the inside the air box.

Now I removed the hose clamps securing the carburetor to the engine so I could turn it around to remove the throttle and choke cables. The rubber hose is connected to a vacuum port on the carburetor.

Now it’s easy to see the throttle and choke springs on the type 94. These are different compared to the R75/5 type 64 carburetor.

Remove Float Bowl, Floats and Float Needle

This is straight forward. Remove the bail holding the float bowl then remove the hinge pin securing the float assembly to the carburetor. Then, remove the float needle from the body of the carburetor. The float needle in this type 94 is different from the one in the R75/5 type 64 carburetor as it has a rubber tip.

Remove Throttle Slide Assembly

This is the procedure for the R75/5 type 64 carburetor. It’s the same for the R100RS type 94 carburetor.

• Remove the Throttle Slide, Diaphragm and Jet Needle

Throttle and Choke Spring Assembly

These pictures show the choke and throttle springs and the throttle bracket assembly.

Remove Jets and “Innards”

This is the procedure for the R75/5 type 64 carburetor. It’s the same for the R100RS type 94 carburetor.

• R75/5 Remove the “Innards” Link

Remove Throttle Assembly and Throttle Shaft

This is the procedure for the R75/5 type 64 carburetor. It’s similar for the R100RS type 94 carburetor but the throttle linkage spring and bracket are different.

• R75/5 Remove the Throttle Linkage, Butterfly and Shaft Link

Remove Choke Assembly

This is the procedure for the R75/5 type 64 carburetor. It’s similar for the R100RS type 94 carburetor but the choke has a spring.

• R75/5 Remove the Choke Assembly Link

Remove Varnish and Carbon

As for the R75/5 project, I soaked the parts in ChemTool B-12 overnight to remove the varnish, carbon and crud.  I used some plastic food storage containers which I got at the local supermarket. These are not affected by the ChemTool.

DANGER:
ChemTool B12 IS VERY NASTY STUFF.  I always wear rubber gloves and glasses when working with it. In fact, it will remove the white paint from the black emblems on the side of the carburetors. Since most of the paint was missing anyway, I didn’t bother to protect the emblems. I’ll repaint them later.

I used 000 steel wool on the brass parts to remove any remaining stains and then used metal polish to return their luster. Then I rinsed them to remove any traces of steel wool.

I use AutoSol aluminum and metal cleaners to clean the castings and AutoSol metal polish to put a shine back on the parts.

Rebuild Kit Parts

The rebuild kit does not contain all the parts I need. I added new jet needles, needle jets, floats, float hinge pins and float needles to the rebuild kit.

The R75/5 write-up has a good review of which size o-ring goes on which part.

• R75/5 Rebuild Kit Parts Link

Here is where the gaskets and o-rings go.

Assemble Carburetor

The detailed procedures for the R75/5 project type 64 carburetor apply very well for the R100RS type 94 carburetor.

• R75/5 Assemble Carburetor Link

Assemble Jets and Mixing Tube

The new o-rings are installed and then the jets and mixing tube are screwed back into the carburetor body. I use some masking tape on the screw threads to protect the o-ring from being cut as I slide it over the body of the jet. By twisting the tape into a taper, it makes it easy to slide the o-ring over the tape. I also use a 90 degree pick to help pull the o-ring until it seats into the groove.

Here is the procedure from the R75/5 project:

• R75/5 Assemble Jets and Mixing Tube Link

I use a small dab of silicone grease on the o-rings. This helps keep them moist and keep water out.

Assemble the Choke

Here is the R75/5 procedure for assembling the choke.

• Assemble the Choke

The key is to ensure the left side parts go together and that the choke disk is oriented the correct way in the choke housing as shown in the pictures. The same procedure is used for the right side carburetor ensuring the right side parts go together. For this reason, I assemble one choke at a time to avoid mixing up the left and right side parts.

Last, I attach the two arms of the choke lever with the nut and washer. It is easy to cross thread the nut and damage the threads on the brass shaft, so be careful.

Assemble Throttle Shaft, Butterfly and Linkage

The procedure is the same as on the R75/5 available here:

• Assemble Throttle Shaft, Butterfly and Linkage

I use a dab of silicone grease on the end of the shaft that fits into the boss in the carburetor to prevent corrosion and keep the shaft lightly lubricated. Note, the o-ring goes in the groove farthest from the threads on the throttle shaft. The groove closest to the threads is where the throttle bracket mounts onto the throttle shaft.

The throttle plate has to align inside the carburetor body while not binding the throttle shaft on the bracket assembly. I align the plate with the holes in the shaft with a pen as a centering tool and then insert the screws with blue loctite until just snug, but I don’t tighten them all the way. Then I insert the throttle bracket into the slot on the throttle shaft and make sure the throttle shaft shaft turns freely. I snug the screws for the bracket into the carburetor body, but don’t tighten them up. Next, I look through the carburetor and move the shaft and throttle plate as needed until there is no gap at the bottom of the plate and the gap on the sides of the plate are even. I look at the plate with a strong light behind the plate to help see the gap clearly.

Then, I tighten the throttle bracket screws and then the throttle plate screws and check that the throttle shaft still turns freely. If it binds, I loosen the bracket screws, adjust the throttle shaft and throttle plate loosening the throttle plate screws if necessary, and then tight all the screws. The throttle shaft must turn with no binding and the throttle plate must have no light showing at the bottom of the plate.

Last, I attach the two arms of the throttle lever with the nut and washer. It is easy to cross thread the nut and damage the threads on the brass shaft, so be careful.

Assemble Throttle Slide and Diaphragm

This procedure is the same as for the R75/5 type 64 carburetor.

• Assemble The Throttle Slide and Diaphragm

It is important to align the tabs in the rubber diaphragm with the slot in the throttle slide (inner diaphragm tab) and the slot in the top of the carburetor housing (outer diaphragm tab). I like to use a little anti-seize on the screws that secure the diaphragm retaining ring into the throttle side.

I attach the carburetor top with the two screws and a dab of anti-seize. Then attach the choke spring in the hole on the tab in the carburetor top.

Assemble Float and Float Needle

This procedure is the same as for the R75/5 type 64 carburetor.

• Assemble Float and Float Needle

The float needle has a small wire retaining loop that goes through a small hole in the stem of the float needle. The loop goes over the tab on the end of the float assembly.

The hinge of the float assembly fits between the posts on the bottom of the carburetor and are secured by the float hinge pin. One side of the hinge pin has serrations so push the pin in from the serration side and then gently, using needle nose pliers, press the hinge pin into the boss of the post. Be careful so you don’t crack or break the post when doing this.

Now, I adjust the tab on the float so the top of the white float is parallel with the bottom of the groove holding the bowl gasket when the tab just touches the stem on the float needle.

Then I attach the wire bail to the holes in the carburetor body and push the bail over the float bowl to secure it.

Repainting the White On the Embossed Part of Emblem

The white paint on the emblem was mostly gone when I got the bike and the Chemtool B12 completely removed it. To repaint it, I used a thin piece of molding and put pin stripping paint on one side of the molding and then pressed it evenly on top of the embossed section of the emblem. If I messed up, which happened several times, I just wiped the paint off with lacquer thinner and tried again. After 6 or 7 tries, the embossed part of the emblem has white paint on it again. Where there were a couple small dents in the embossed areas that didn’t fill with paint, I used a very small artists brush and carefully filled them in. Any slight drips or spots on the emblem were easy to remove after the paint tried by carefully using the dull side of an Exacto knife blade to scrape them off the black emblem.

What Done Looks Like

Here is the rebuilt and refinished carburetor and what it looks like mounted on the bike.

Revisions

2019-11-22  Edits and typos.
2020-08-26  Fixed bad link to R75/5 Assemble The Chock chapter.
2020-09-02  Fixed multiple bad links to R75/5 carburetor rebuild material.
2023-12-22  Added Video section on rebuilding Bing CV Carburetors.