Idle air adjustment on the /2 BMW motorcycle Bing carbs
This page is about the BMW motorcycles from the mid-’50s thru 1969. The R26, R27, R50, R60, R69, R50/2, R50S, R60/2, R60US, R69S, and the R69US. I think other websites deal with the/5 and later procedures very well.
The Bing carb is a very simple device. They are made from a casting. The castings vary a bit, as well as the engines. No two adjust exactly alike. One side may require 1 1/2 turns on the air adjustment, and the other side 2 turns out. Don’t fret over this as long as all else seems correct and the carb adjustment responds correctly. This page is not a full discussion of the Bing carb adjustment. Get the handbook from Bing and read it. Click here to see an exploded view of your carb. Here are only a few hints of what we learned. This assumes the engine has good compression and properly adjusted valves, and the ignition timing/advance works well. Read about the many considerations and limitations in tuning a BMW twin.
Doing this as a business, I had to develop a better and faster way. I found that I could adjust the carburetor only about 1/2 of the time, and everything was OK. For the other half, no amount of adjustment would get it right. I learned that the adjustment procedure was a diagnostic procedure that would reveal a problem(s). Often, one side, or both, would have some anomaly. Don’t accept some compromise; instead, fix the problem.
An important initial check
I didn’t find this defect often in the old days, but it must be more common by now. While recently working on a customer’s bike in my shop, I was surprised to find this defect. While I am not pleased to find it, it serves as an example in this case.
The problem is that the carb idles too fast and refuses to adjust properly. This is what you might find.
A gray metal part is cast into the carb body for holding the main jet mount. Over time it can move upwards in the casting. When this happens, it will also hold the slide up too far. It would be nice to have an actual measurement, and I tried to see where I could measure from, but it isn’t so simple.
Another view of the problem
Here you can barely see the serrations on the body of the jet holder. These serrations should not be visible. In this particular case, the serrations are not showing very much. I have seen many that were more visible. We had success with just shoving it back down and watching it over time.
This photo shows me fixing the main jet holder. I have used a deep well 8 mm socket by Craftsman on a 1/4″ extension. The size of the socket is only important in that it “fits” the job. The diameter must be small enough to fit in and not touch the carb body. The socket inner size must be large enough to go over the main jet holder. It only took a few gentle taps to shove it down. I hope it stays and will suggest to the owner to check it from time to time.
If it moves up again, I suggest you consider using a bit of epoxy at the edge. You might shove it back down and then peen some of the aluminum over. That may not work.
I only remember one carb where that part came loose, and we had to replace the carb body. We did not investigate an epoxy solution. By this late date, 2017, I would expect this to be a more common problem. Check your carbs for this failure.
This is another carb. The main jet holder is fully seated, but one can see that it is damaged, and maybe it is from someone hammering it back down. Because the jet holder is all of the ways down, the idle stop screw tip is lower than the one in the other carb shown above. Another issue can be seen. The scratches in the front part of the bore are common and might cause the slide to stick.
When I see a carb with this much wrong, I am quite concerned about how well I will be able to adjust it.
BMW motorcycle carb tuning procedures, by ear
Make sure that the engine has properly adjusted valves and timing and is fully warmed up.
Make sure that you have the choke off.
Make sure the intake tubes are fully mounted.
On the sports models, ensure that the rubber sealing ring on the intake tube to the carb has not swelled up so much that it forces the intake backward.
You should already have found that the difference in timing between cylinders is within spec.
Make sure that you know the cylinder compression. If it is low, then you won’t find that the carb responds well.
I adjust one carb enough to get it “in the ballpark” and then the other one. Once both are close, by my method, they will be idling too fast, and the mixture may be off a tiny bit. Then I fine-tune each while both sides are running. This is the fastest and most accurate way that I have found.
Don’t bother with pulling a plug wire on the “other” side, as suggested in the owner’s manual. For that method to work, the engine must be in very good condition, and the running side must already have the carb adjusted well. Once you are finished with both sides, as a final test, you may remove one plug wire at a time to see if it will idle on only one side. The R50 and R60 should idle well on only one side, but the sports models are a bit less likely to do it at a low rpm. (Never pull a plug wire while running, on a /5 or later.)
Slightly back off the idle screw on the other side. That takes it out of the circuit but still assists a bit to keep it running. You might think of it as running on 1 1/2 cylinders. A bike that dies when you pull a plug may continue to run if you just run the idle stop screw down a bit. You might need to increase, very slightly, the idle screw on your target carb to keep it running by bringing it back to about the same rpm. The slight help of the “barely running side” is just enough to keep it running, but not so much as to get in the way of adjusting the idle mixture on the target carb. If it runs primarily on one cylinder, then that still makes the adjustment changes very obvious.
There is a disadvantage to this. You are adjusting each side separately, but it isn’t quite right when both are put together. When it is running on one cylinder, the conditions are so changed that this perfect adjustment just reached isn’t quite perfect for when it is running on both cylinders. You may want to “touch it up a bit” when you have both sides adjusted separately. Here are a few factors. The rpm has dropped quite a bit, and adjustments are for that rpm. The ideal adjustments at normal idle rpm are different than at the lower one with only one cylinder running. Remember the crossover on the exhaust system? One side, not running, may change the characteristics enough to alter the settings. This sounds complicated.
First, set the mixture adjustment. Start with the screw at the recommended setting for your model. One turn out should be OK. Rotate the mixture screw slowly in or out 1/4 or 1/2, turn at a time, and wait to see if the rpm changes. Notice where it starts to stumble when screwing it in and also out. Those two points are usually a turn or so apart but may vary a lot. About 1/2 way is in the ballpark. Set it for maximum rpm and then go in (richer on the /2) about another 1/8th to 1/4 turn. That last 1/4 turn makes it slightly richer, but any rpm decrease should be small or nonexistent.
You may find that your mixture adjustments will increase the rpm a lot. Then adjust the idle screw out a bit to a lower rpm. You only want the rpm just high enough that it keeps running. Adjusting it at a low rpm is more sensitive and, therefore, more accurate. After both sides are done, adjust both idle screws to where it runs equally on both sides. That is best determined by going to the rear of the motorcycle and putting your head near the rear tire. This way, you hear each side run in stereo. You will easily hear any difference.
I would then go to my “idle mixture test.” With it idling as best as it can be, run one idle speed screw up about 1/2 turn at a time. Each 1/2 turn should increase rpm by nearly an equal amount. If there is no change, it starts stumbling, or it runs slower, then that is what I call a “flat spot.
No increase in rpm is a minor flat spot
A stumble is a medium flat spot.
If the rpm drops, that is a seriously major flat spot.
The two most common reasons for a flat spot are plugged idle gas jet or an intake leak. The gas jet, usually a #35, can be removed, carefully inspected, and cleaned. It can plug up again immediately. It may take several removal and cleanings to get it to stay clean.
All /2 BMW Bing carbs were delivered to the USA with a # 35 jet. I saw many European delivery bikes with a # 30 jet. They wouldn’t adjust properly with our fuel in the USA. I changed them to a # 35, and that would usually fix it.
I found it necessary to use a round screwdriver that had a blade that exactly fit the slot in the idle jet. Some screwdrivers have a square shank, so don’t use them. The square edge can “scrape” off some of the soft metal, and that can clog up the jet. Get the round type. Grind the screwdriver tip to fit that slot perfectly. A perfect fit is important. The round ones still have square edges that stick out where they were pressed into shape at the tip. Grind these square edges off so that they are smooth. I suggest that you use some shrink wrap to protect the sides of the screwdriver from touching the side of the carb. Insert the screwdriver carefully into the cavity, and don’t allow it to touch the sides. Carefully “feel” for the slot and move it back and forth to make sure that the screwdriver is well centered in the slot. You do not want it touching the side.
An intake leak is usually due to warped carb flanges. That is my article on how to fix the warped flanges yourself.
A way to “cheat” the flat spot out is to run the idle adjustment up to the flat spot. You may notice that the engine stumbles. Adjust the idle air mixture screw inwards a bit. This will make it run richer and may clear the flat spot. This isn’t the right way to do it, but it may do the trick until you get around to doing it correctly. You now have a situation where the idle is too rich, but it is correct at a slightly higher rpm than the former flat spot. I saw this most often in the late /2 that had the “lever top” type of float valve.
The sidecar float chamber
You may be asking, “What is a sidecar float chamber?” That is simple. On a solo bike, the line of gravity is always straight down through the motorcycle’s centerline as you lean right and left through curves. On a sidecar bike, as one goes around a right curve, the force shoves the fuel in the right carb towards the center of the motorcycle. The centrifugal force will starve the main jet. Bing has a float chamber on the “outside” of the carb to supply fuel while going through a curve. During a long curve, BMW found that the chamber wasn’t large enough and would finally use up the fuel and starve out. In 1963, Bing made a different casting to hold more fuel for the long curves.
This photo shows the older type with the small sidecar float chamber. It is the long vertical round part in the center of the photo.
Here is the bottom view, and you can see the 4 threads showing on the mixture screw. The slot screw cap on the left is smaller than the hex main jet cover.
This photo shows the 1963 and later type sidecar float chamber. It is about twice as fat.
Here you can see that the slot screw cap that covers the chamber is far larger than the main jet cover. This main jet cap is famous for falling out. Carry a spare and gasket. If you don’t have one, then find a stick on the side of the road and carve it to fit. Then shove it in to plug off the fuel until you get a replacement.
In 1968, the carb float bowl tops and some less obvious items were changed. We call those “lever tops” because of the lever action on the float valve. The nasty symptom is that no adjustment will get it right. I found that changing to a # 40 idle air jet fixed it. In a few cases, I had to go to a # 45 to get it to adjust correctly. You may need a different size jet on each side. Don’t worry about it. Just make it idle (and lift off idle) smoothly. (In 1968, the flywheel was slightly lighter too. The frame no longer had sidecar lugs; without a sidecar, it didn’t need the heavy flywheel. This light flywheel makes it slightly harder to only idle on one side.
The sports models R69, R69S, and R50S had a slightly different gasket between the carb and head. It has a metal sealing ring and provides a much better intake seal. We saw far fewer intake leaks with this system.
This photo shows the sports bike intake gasket and ring. The ring is wider than the thickness of the gasket. The ring goes on the inside of the gasket. The carb body and head are slightly relieved to take this wide ring that sticks out on each side a bit.
The intake tube on the sports models is also different. This tube has a rubber seal hidden inside the bulge at the end of the air tube. There is also a metal part there. This rubber seal causes a lot of tuning problems. It absorbs fuel and expands. As it expands, it shoves the chrome tube off of the carb. The chrome air intake tube can slide back one millimeter and be OK, but any more, and you risk an intake leak. Some air will leak past the rubber seal and disturb the flow pattern. The carb can’t be properly adjusted.
The solution is simple, remove the rubber sealing ring and replace it. Don’t throw the old expanded ones away. Leave them sitting out in the air for a couple of weeks, and they will shrink back down. Then recycle them every few months for proper carb operation. Do not lube the rubber seal with oil, as it will slide off even faster. I used a few drops of fuel, and it evaporated away, so the chrome intake tube sticks on tightly.
Idle air mixture adjustment
Don’t be concerned if your carb requires a different setting than some authority dictates. The main thing to know is that you should find 2 things. The idle air adjustment should have a peak rpm setting.
1. If you adjust the screw more, the rpm starts dropping.
2. If you adjust it out more than the max rpm, it should again drop in rpm. If you find that the best rpm is with the screw all the way in, then you have a problem that needs attention.
After you have tried the “idle mixture test” a few times, you will learn what is normal. It is a fast and accurate way of diagnosing if the carb can be adjusted or if more work needs to be done first. Try it on your now perfectly operating carbs to learn. You are now thinking, “This guy wants me to screw up my carbs,” and I don’t blame you. You should count the turn or fraction of a turn, in or out. That way, you can always return it to where it was.
Be sure to record your symptoms and solutions in your BMW logbook.
Updated 15 July 2022