1983 R100RS Rebuild: Bing Type 94 Carburetor & Karcoma Petcock Rebuild/Refinish

You know, being retired just seems to leave no time to work in the shop. I end up doing all sorts of things and the next thing you know, time for bed.  Anyway, I finally wrote up the Bing carburetor rebuild/refinish work I did on the 1983 R100RS project.

When I rebuilt a 1973 BMW R75/5, I covered the procedure I used to rebuild the Bing type 64 carburetors, which are 32 mm, and the Karcoma petcocks. The R100RS has type 94 carburetors that are 40 mm and the Karcoma petcock outlet is 90 degrees from the inlet instead of directly underneath the inlet as for the R75/5. After 10 years, the designs are essentially the same, but there are some differences which I cover in the procedure write-up.

Here is the link to the R100RS Bing type 94 rebuild/refinish procedures.

The R100RS write-up contains links to the R75/5 procedures for the Bing type 64 carburetor and notes any differences or changes in how I did the work on the R100RS carburetors. Here is the link to the R75/5 type 64 carburetor rebuild/refinish procedures.

The combination of these two write-ups covers the type 64, 32 mm (64/32/10, right side and 64/32/9, left side) and the R100RS type 94, 40 mm (94/40/114, right and 94/40/113, left) carburetors.

I hope this material makes it straight forward for folks to rebuild these carburetors who may have been afraid to tackle a carburetor rebuild. Here are some before and after pictures of the carburetors.

Before Rebuild/Refinish

Before-Left Carburetor

Before-R100RS Type 94 Left Carburetor

Rusty Screws and Cap on Dome

Engine Side of Carburetor

Grunge and Fuel Stains

Float Bowl Removed

Float Bowl Removed, Missing Paint on Emblem

After Rebuild/Refinish

Engine Side of Restored / Rebuilt Carburetor

Engine Side of Restored / Rebuilt Carburetor

Top Side of Restored / Rebuilt Carburetor

Top Side of Restored / Rebuilt Carburetor

Outlet Side of of Restored / Rebuilt Carburetor

Outlet Side of of Restored / Rebuilt Carburetor

Carburetor Mounted - Clean

After-R100RS Type 94 Rebuilt and Refinished Carburetor

16 thoughts on “1983 R100RS Rebuild: Bing Type 94 Carburetor & Karcoma Petcock Rebuild/Refinish

  1. Excellent info, Brook! Thanks for investing the time and energy in documenting your work for others! Your posts have been helpful and instructive for me.

    • Hi John,

      Thanks. I hope this encourages folks with problem type 94 carburetors to do their own rebuild.

      Best.
      Brook.

  2. Thank you very much for this valuable information. I have a 1975 R75/6 & a 1977 R100RS and have owned both since the mid 80’s. They run great but carbs should be overhauled. Thankfully they’re finally selling ethanol free gas at many of our Upstate New York Stewart’s Shops and the bikes love it. Thanks again!

    • Hi Carl,

      Yes, the ethanol doesn’t seem to sit well with these vintage carburetors. I wish you success on rebuilding the carbs on your two machines.

      Best
      Brook.

  3. Brook: Once again you are providing a very valuable resource to keep these old bikes running in tip top shape. The carb rebuilds are relatively simply and no one should hesitate to undertake them, especially with your guidance. I’ve got a spare set and simply changing the jets allows switching between the R 80 RT an the R 75/6. Thanks for your support for the entire airhead community!

    • Hi Dale,

      Now I know where to go for an emergency set of type 64 carburetors 😉 And, thanks for the kind words.

      Best.
      Brook.

  4. Is there a way I can send you a photo of my piston orientation? Your picture is different than my piston. There are two scarfed pipes on mine that I do not see in your picture. During the carb kit rebuild I installed new Floats and Butterfly Shaft screws; new Jet Needles set at notch number two. I wonder if my piston is installed 180 degrees wrong! I have had my R100 cylinders plated with NikaSeal and had the heads rebuilt. I rebuilt the carbs; installed new points and condenser, new solid state volt regulator, a new Alpha coil, a new ThunderChild Diode board and many other new features. It starts right up BUT goes to a high RPM and cuts out. The cables and points are adjusted and gapped correctly. Rotating the dwell cannot bring it down enough to idle. I suspected a vacuum leak but cannot find one. What can you tell me about the scarfed tubes my Bing 40 mm carb pistons have that I do not see in your picture? Thanks for your great web page, time, efforts and advice.
    It looks like there are more than one “Dale” writing to you.

    • Hi Dale,

      Thanks for dropping by. I wonder if you looked at the full write-up with the picture of the piston or just the blog post which is only a summary?
      http://brook.reams.me/bmw-motorcyle-rebuilds/1983-bmw-r100rs-rebuild-project-index/13-bmw-r100rs-rebuild-bing-carburetors-karcoma-petcocks/
      https://www.flickr.com/photos/brook-reams/16744550019

      I’m not sure what you mean by “scarf”, but looking at the picture above, you can confirm what the piston should look like.

      Note that the rubber diaphragm has tabs in it on the inner circumference of the hole and the outer circumference. The inner tab mates with a slot in the top of the piston and the outer tab mates with a slot in the top of the carburetor body. Check this to be sure they haven’t slipped out of the slots or the piston will not be in the correct alignment in the bore of the carburetor.

      Another thing to investigate is the choke assembly. The parts are right handed and left handed and mixing them creates problems. Assembling them backwards creates problems as well.

      Be sure that the choke lever adjustment allows the choke lever on the carburetor body to be all the way down. A slight elevation in the choke lever on the carburetor body won’t help things.

      If you pull the plastic inlet tube from the air box to the carburetor, look inside and be sure that both pistons are all the way down to the bottom when the throttle on the handlebar is closed. Check that they both move up smoothly and return to the bottom smoothly.

      Those are some things to verify.

      I hope this helps.

      Best.
      Brook.

      • Thanks, you were right, I was in a blog instead of a full write up. After going through the full write up and photos, I brought in my carbs and compared; still nothing wrong with what I have done that I have found. The engine starts right up but still has the high RPM’s. I have gone through the whole set up; carbs, cables, points, condenser, all with new parts several times. Sooner or later I will stumble on what I have done wrong and then I will write again… and confess! LOL It is there somewhere and I want to be the one that finds it, not some repair shop. The learning curve, got to love it.

        • Dale,

          Some things to look at:
          1. Make sure the throttle butterfly on each carb was installed correctly. One side has a slight bevel on it, the other doesn’t. The bevel side should be “inside” the throat of the carb.

          2. Check that the shaft the butterfly mounts into freely rotates and that the butterfly can close all the way. It can bind if not assembled correctly.

          3. Ensure that the o-ring on the throttle shaft is not damaged, twisted and in FACT is on the shaft. DAMHIK 🙂

          4. Check the advance unit under the front cover to be sure it is working correctly. If the weights can’t retract due to weak springs, it will keep the idle high. A way to test them is to drag the clutch in 1st gear to bring the RPM down and then pull the clutch all the way in. If the speed stays down, but starts climbing again, weak springs are a likely cause. BE CAREFUL. DON’T ALLOW THE BIKE TO TAKE OFF IN 1ST GEAR.

          5. Verify that both jet needles are in the same position. It is easy to get them one notch off, and/or to have them set in the wrong position. This may cause increased idle.

          6. Verify that IN FACT, the two needle jets (the orifice the jet needles go inside of) are the same number jet and the holes look to be the same size. There have been reports of incorrectly made needle jets.

          I hope this helps.

          Best.
          Brook.

          • Brook,
            In response to your suggestions, and thank you for making them; the butterfly bevel is correct, the butterfly shaft freely rotates and closes all the way, and the throttle shaft O-ring is new and not damaged. I examined the Advance Unit on my kitchen table and the new Points are correctly gapped with the Bearing Plate Support in place. The weights do retract, springs are good. I have not tried the test with the clutch in first gear because I have not been able to break in the engine yet and do not want to “race” the RPM’s for any length of time for such testing. I need to find the issue, correct it and then break in the engine.
            Perhaps this is something: According to BING there are Jet Needles of different sizes and the original size was #341. I bought new Jet Needles but they are only referred to on the receipt as part number 13000335321 and called a Slide Needle 40mm. I trust the BeemerShop not to sell “incorrectly made” parts of any kind. The new Jet Needles are both installed into the pistons on the second notch. The # 2.66 Needle Jets are the original needle jets but were in good running order before the rebuild. The reason for replacing both Jet Needles was, I mistakenly thought they were supposed to come to a sharp point which they do not. Since I had the new ones, which are exactly like the originals, by looking with my naked eye, I put the new ones in. Perhaps there was a different size Jet Needle sent to me (an example would be a number like #371?) Could that make it have a higher RPM than idle speed?
            Thank you so much for your help.
            Best Regards,
            Dale

          • Hi Dale,

            Interesting. At this point, when it seems like I checked everything and everything checks out ok, I start challenging my assumption. So, I list what I assumed I know, and then list how to “prove” that my assumption is WRONG. That’s the key, choose to prove an assumption is wrong, not to prove it is correct. That changes how you approach the problem.

            I will note that one of the problems with jet needles and needle jets is that the needle vibrates inside the needle jet and this can widen the jet and narrow the needle increasing the bore and the fuel flow. So, when I do a complete rebuild, I always replace both of them. If I replace only the needle, I may have an enlarged jet. You can not visually see these differences, so visual inspection is not reliable IMHO.

            Yes, dealerships try to provide correct parts, but please note that BMW prices have been steadily increasing and that has increased the pressure to source parts not actually from BMW; and further, BMW itself is now suppling parts from various sources, some of which are not up to snuff. I’m not saying you have bad parts, just challenging your assumption 🙂

            You don’t mention the chokes, aka, enrichers. If the disk goes in wrong (rotated incorrectly, or left swapped with right) that will affect the idle at startup. And, based on what you said about not wanting to let the engine run at high RPM since it’s just been rebuilt, then you likely are running a cold engine, so the choke may be involved in causing the high RPM condition.

            A thought came to me to remove the idle air mixture screw and see what happens. Or, screw it out 4 or 5 turns and see what happens. What I’m fishing for is if there is something odd about the idle circuit condition. If you screw the air mixture screw out a lot, then I would expect the engine to not start or run at very low speed. So, I’m “fishing” to see if I can prove the problem is somewhere in the idle circuit.

            You get the idea, I think, about what you can try to do to prove your assumptions are WRONG. Sometimes that approach locates the problem(s). Yes, I added an “s” because sometimes, it’s multiple problems that are the cause and not just one.

            I wish you the best on this.

            Best.
            Brook.

          • Hey Brook,
            Thanks again for your latest response, it made me think from a different POV. Here is what happened; I stood there looking at the machine going over in my mind everything I had done during this thousands of dollars upgrade. The answer was not in the present but rather in the past. One of the many things I did was all the way back to 2006 when I first bought the bike. It had the large US handlebars with huge spongy grips and original bench seat. To say the least it just was not my style so I ordered “Made in Germany” European Bars and the shorter cables. Key words here are “shorter cables”. Then I replaced the two person bench seat with a solo seat and removed the rear pegs. It put me leaning a little bit forward which I never liked. It is NOT a café bike, let me make that very clear. When I started this major upgrade this summer I also removed the steering damper because I never used it and it just looked too dated for my taste. When I was waiting for the cylinders and heads, I bought lots of parts to put everything back together again. A couple of the items I bought were a stem nut without a hole and risers. Key word here is “risers”. You know where I am going with this. 😉 I just dialed out my shorter cables as far as they go, started the engine and it was just a bit of a high RPM. After turning off the choke and adjusting the dwell, it idles at a perfect RPM. Thank you for making me look at it from a different angle and stop assuming it was just one thing. I was wrong to think it had to be only the carbs or the dwell. I still have the old longer US handlebar cables and might try them or create custom cables.
            All is well that ends well.
            Again, Thanks for your help,

            Dale

          • Dale,

            Fantastic!!!! I’ve found many times that changing my “point of view” changes my perceptions and all of a sudden, something interesting becomes visible that previously I wasn’t able to see. It’s the “Eureka” moment, isn’t it?

            I’m glad you have gotten this R100RS back on the road.

            Best.
            Brook Reams

          • In the manual BING CARBURATORS page 9 figure 11 paragraph 3 states the following. The vacuum in the venturi acts on top of the diaphram and the plunger (i.e. Air Valve) via a bore (U) in the plunger and attempts to lift the plunger against its own weight and spring.
            Figure 11 cut-away places the bore (U) on the same side of the venturi as the throttle. Can that be correct? I look at your photos and it look like they are 180 degree different than the manual.
            Where is (D)?
            Why is the VTO not shown?
            Why is the (GRS) Mixing Screw not shown?
            The more I study the Bing manual, the more confused I become. Is the Bing video as confusing as the manual?
            I rebuilt my Type 94/40mm carbs like your web page shows for the CV and they work fine but when I study to understand how the carburetors really do work, it is an unfinished puzzle. Thank you again for your time and efforts.

          • Hi Dale,

            I’m pleased my description of how to rebuild the BING carburetors helped you with yours.

            I have read their book, a couple times. As is the case with technical publications (mine included) things get left out, misidentified or explained incorrectly or in an ambiguous way. I haven’t watched their video, so I can’t make any comment.

            I understand the theory of operation of the CV carburetor. The vacuum pressure on top of the diaphragm acting on its area overcomes the weight of the slide (aka, plunger) and jet needle. The vacuum pressure changes with altitude, becoming weaker as the air pressure drops. So the force pulling the slide up is less and it doesn’t rise as far. That provides some active compensation for higher altitudes by reducing the amount of fuel pulled from the float bowl.

            Therefore, the passage between the chamber above the diaphragm and the inlet of the carburetor body should be at, or close to, the narrowest point of the venturi. Since the slide moves the jet needle and the jet needle is in the center of the venturi, the holes in the bottom of the slide are at the lowest pressure point. So, that’s how the vacuum pressure in the venturi ends up above the diaphragm.

            I hope that helps.

            Best.
            Brook.

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