1973 R75/5 Problem With One Carburetor Running Rich

I found the right cylinder was running rich, but not the left.

Right Spark Plug-Can You Say SOOTY

Right Spark Plug-Can You Say SOOTY

So I started from the beginning.

  • I checked head torque at 26 Ft/Lbs. Nothing was loose
  • I checked the valve lash. Spot on.
  • I checked the jet needle position. Correct at #3 just the same as on the left.
  • I checked the main jet and needle jet sizes. Correct sizes and matched the ones on the left. O-rings are in good shape and not torn or brittle.
  • I checked fuel depth in the bowls. They are the same on both float bowls; between 19 and 20 mm. The fuel level is correct on both sides.
  • I checked the timing. It’s spot on and there is no ghost image. The timing advances smoothly.
  • I switched the spark plugs and rode the bike for 50 miles. The original left plug, now in the right cylinder, is sooty and the original right plug, now in the left cylinder, is fine. So, there is no issue with the plug, plug cap or plug wire.

If it’s sooty, it’s rich. So, what’s going on that causes only the right side to run rich?

The, valves, timing, jets, needles, spark plugs, wires and caps are fine. There is no obstruction in the air tube feeding the right carburetor. The crank case breather isn’t showing any more oil than normal, so it’s not getting a lot of oil mist into the right carburetor. I mulled this problem over for several days with no new insight as to what to try next.

A couple days later I was at coffee with some friends and was chatting about this problem when an idea hit me. What if the slide isn’t going all the way up at full throttle? If that’s the case, the vacuum will be higher than normal at higher engine speeds. A higher vacuum means the fuel delivery is going to be higher than normal but the air volume will be lower than normal. That certainly would create a too rich condition and sooty plugs on one side, but not the other.

I take the air tubes off both carburetors and push the slide up on the right side. It won’t go all the way up.

Right Slide Can't Go All the Way

The left one goes all the way up.

Left Slide Goes All the Way

So, my hypothesis is correct. The rich condition is caused by the right slide sticking.

But what’s the cause of that?

I updated these carburetors with springs (not originally used on the /5 carburetor, but added to later versions). These help smooth out the transition from idle to mid-range.

Spring Installed

As I look at the spring, it looks like it can get caught on a pin that protrudes inside the air well the spring fits inside of.

Pin Inside Slide Where Spring Goes

I can get the spring to hang up on that pin. When I do, the spring is cocked and then snags on the split washer under the screw that holds the plate onto the diaphragm. That stops the slide from moving.

Diaphragm Screw & Lock Washer

I think this “cussedness of inanimate objects” is what stopped the slide from going all the way up.

I install the spring and make sure it isn’t hung up on the pin. I also loosen the screws holding the plate that secures the diaphragm and push the split washers outward so they aren’t protruding into the air well.

After I put the carburetor back together, the slide moves freely. I’ll take the bike for a ride when it warms up and see if I have fixed the problem. I’ve grown accustomed to not concluding I’ve fixed something until I take a longer ride and the problem doesn’t reappear. πŸ™‚

19 thoughts on “1973 R75/5 Problem With One Carburetor Running Rich

    • Don,

      You’re welcome. This was an interesting situation. I hope my diagnosis turns out correct. πŸ™‚

      Best.
      Brook.

    • Dom,

      Thanks. This is an example where focusing hard on the problem and trying everything you can think of, but not solving the problem, means it’s time to go away from it and stop thinking. Then, after a bit, come back to the problem and explain what you know to a disinterested third party. Frequently, you will think of something you hadn’t thought of before, and that leads to the solution.

      Best.
      Brook.

  1. By the sounds of things, you are becoming an Air Head Guru. A diagnosis like that is knowing your machine quite well. Good job! If that does not work tho, check the diaphragm for cracks.
    Thank you for all your great videos and pic’s to help us get the confidence of working on our own bike.

    • Hi Mike,

      Thank you for the kind words. I did inspect the diaphragm and it is not damaged. You will note that a damaged diaphragm can not cause the slide to stick as mine did. So I’m hopeful I found the cause of the sticky slide.

      Best.
      Brook.

    • Hi Adam,

      Well, it certain was a problem, but there were other tweaks I needed to do.

      I also found that the slide shaft was sticking inside the carb top. I suspect the top maybe warped a little bit. I used 600 wet/dry paper and polished the shaft and then carefully tightened the top screws evenly. That seems to keep the slide moving smoothly in the sleeve in the carburetor top. I have another top and will likely put that in and see if that eliminates the tendency of the slide shaft to bind up in the top.

      I live a bit above a mile high, so in the end, I put in smaller main jets and dropped the jet needle to the #2 position. Those changes and the stuck slide seem to have contributed to the rich condition. I hope to put a couple hundred miles on the bike Saturday riding in the mountains and that will give me an opportunity to verify how well these changes work.

      Best.
      Brook.

    • Hi Anders,

      I would suspect a plugged idle jet, plugged idle jet passages, or o-rings on the idle jet and/or idle mixture adjusting screw are crumbling. You can remove the jet and mixture screws to inspect the o-ring condition. Then use carburetor cleaner to spray into the holes to blow out the passages. WEAR PROTECTIVE GOGGLES, FACE MASK AND RUBBER GLOVES. Carburetor cleaners are very toxic.

      I hope this helps.

      Best.
      Brook.

      • Brook, I’m trying to figure out why my R90/6 runs unevenly, alomost sputters at constant velocity/rpm. I did see that the back of the mufflers show black discoloration, especially on the right side. Acceleration is good though. Chris

        • Christopher,

          Start at the beginning and work your way towards the carburetors.
          1. Verify that the valve clearances are correct.
          2. Verify that the ignition timing is correct.
          3. Verify the fuel level is the same in both float bowls.
          – Remove the bowl. With fuel line attached push up the float until fuel stops flowing.
          * The bottom edge of the float should be parallel with the float bowl seam in the carburetor that the bowl gasket fits into
          4. Verify the float bowl gasket is not cracked or broken.
          5. Verify there is fuel in the small well in the corner of the float bowl
          6. Remove the top of the carburetor and inspect the rubber diaphragm
          – If torn or it seems very flabby, replace it (and the other one at the same time)
          7. With engine running at idle, check for air leaks in the rubber connector between the carburetor and head.
          – Spray some carb cleaner or brake cleaner on:
          * the rubber tube between the carb and head
          * around the inside of the carburetor where the butterfly shaft fits
          – If RPM changes you have an air leak you need to fix.
          8. Balance the carburetors at idle and at 1500-1800 RPM
          9. If the problem persists if maybe time for a carburetor rebuild. You will replace all O-rings and gaskets and the jet needle and needle jet. I show that work for both size carburetors on my site:
          13 BMW 1973 R75/5 Rebuild & Refinish Bing Type 64/32 Carburetors & Karcoma Petcocks
          13 BMW 1977 R100RS Rebuild & Refinish Bing Type 94/40 Carburetors

          I hope that helps.

          Best.
          Brook.

          • Thank you, Brook. I just checked the spark plugs. The left was light grey and dry, the right side was very black and dirty. What does that tell us?

          • Christopher,

            It’s running rich. But why is the question. Follow my diagnostic procedure and you will likely find the cause.

            Best.
            Brook.

          • Christopher,

            Well, sure, why not ensure both sides (engine and carbs) are adjusted correctly.

            Best.
            Brook.

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