..."Very slight, perhaps a couple thousandths, up and down movement of jet needle in slide."...

My next move would be to get to the bottom of that issue. Then I would get new needles and needle jets and install the needles using the position indicated for your carburetors in the Bing book. The needles have 3 or 4 indexing grooves at the top where they are fitted to the slides. The groove on the needle can develop wear which would result in less than precise positioning. The grooves have a sort of stair step feature. You can remove the needle by slowly spinning it between your finger and thumb while gently pulling downward. Installation is the same only pushing in gently.

Will do the needle and jet. Was leaning in that direction but didn't want to spend the money needlessly. Had worked on many touring bikes with high mileage before and replaced very few worn needles and it seemed like the ones I did had noticeable damage. Then again this 40 mm Bing is one big carb and I am unfamiliar with the nuances.

I have seen advise being given to go with the Euro needle and needle jet. I will be doing a lot of interstate running in west Texas for hours at a time close to 90 mph.

O.K. Bret. First off, I have never owned or worked on a bike with 40mm carburetors, I would go with the assumption that they are similar in function to the type 64 carbs on the smaller displacement motors.
Second; if you do not already have the Bing carburetor book, buy a copy from Bing Agency. I find it to be very handy when working on carburetors. It has a jetting chart for carburetors by their numbers and a very good exploded view illustration.
I don't have an opinion about european jetting as compared to USA except that it would follow that USA jetting produces a leaner air/fuel mixture with regard to emission compliance .

Bret -
► I agree with the new needle and new needle jet idea on a bike with those kinds of miles. However, some of the 40mm carbs don't allow you to actually see the notch the needle clip falls into. (In effect you are working "blind".) Therefore, you need to take a measurement of the exposed needle with a caliper BEFORE disassembly, then install BOTH new needles to the same exposed length.

On a recent R75 repair that displayed similar issues, I found that the owner had simply not installed the needles to the same exposed length !!

► While the slides are out it will be mandatory to inspect the diaphragms for holes. Hold the slide up to the light and stretch the diaphragm tenderly as an inspection.

► The float needles are viton-tipped, and that soft elastomer will take a set after several years. Look for a divot or ring around the sealing surface. Additionally, the tip will collect a coating left by old fuel which you cannot see, but which will allow fuel leaks at the float seat. If you have any doubts about the float needles, then by all means replace both float needles.

► Lastly, if you see any discoloration in the bottom of the float bowl, then the bike has been allowed to sit for some time with fuel in the system... which is NOT a good thing. I highly suggest use of a product called StarTron to cleanse the entire fuel system. Not just the float bowls, but the inside of the tank, hoses, and petcocks. This is important.

Will do the needle and jet. Was leaning in that direction but didn't want to spend the money needlessly. Had worked on many touring bikes with high mileage before and replaced very few worn needles and it seemed like the ones I did had noticeable damage.

It's been said that the cheapest thing on a BMW is the rider !! 😛 I assure you that no one wants to spend money needlessly, but there are times when you need to spend some money simply as insurance, just so you don't end up doing the job 15 times. One of those jobs is carb work. Shrinkage of the float bowl gaskets will always mandate new gaskets. Rubber parts of unknown age (intake hoses, diaphragms, float needles, etc) should all be replaced. In order to run correctly, both cylinders need to run as close to the same as possible. The only way to achieve that is to start with some new carb parts.

I assure you that due to the high price of labor, that if this bike ends up going to a shop, the first thing a real mechanic will do is replace all the mentioned parts as a base-line. The reason is that at $75 per hour, it's not worth his time to fiddle around with guessing about the condition of these parts or spending the time to look at them under a microscope. Simply put, it would cost you $75 to find out that your $15 float needle is OK. It's simply false economics.

I have seen advise being given to go with the Euro needle and needle jet. I will be doing a lot of interstate running in west Texas for hours at a time close to 90 mph.

Be advised that 1) the hotter ambient air temps of Texas may need to be offset with a slightly richer fuel mixture. 2) Also that original jetting for this bike does not apply since ethanol fuels did not exist at the time of manufacture. 3) Ethanol contains less energy than gasoline, therefore if you hope to cruise at 90mph, then you may have to allow more fuel to enter the engine.

All the best !

Fought the same problem a while back. O rings from flaps were almost right but not exactly. Try adjusting the mixture screw. If it makes no difference or if it is far away from the 1 1/2 turns out, you may have a blockage in the air passage. Clymers has a picture of the carb showing this opening. Idle mixture is in play for more than just idle.
Bing book says the 45 is correct and needle should be in position 2. Carb cleaner sprayed in the intake side with both the idle jet and the screw removed opened mine. Also had a broken O ring on the idle jet earlier. O rings from a Bing kit broke while those bought from a dealer went on easily and appear to be good. Main and needle jets can vary with different carbs on the R100, but if the left is good, the right should also be good with the same jets and under the same conditions. When both sides act the same then you can start changing needle from 2 to 3 and then playing with jets but making changes to stock settings before finding the reason for rich running on one side is going to waste a lot of time. Bobs sells a kit of just o rings and gaskets for rebuild or sells individual parts for carbs. Shipping may cost more than the cost of an o ring. If there is a dealer nearby, they may have or can order needed parts.

Bob

Fought the same problem a while back. O rings from flaps were almost right but not exactly. Try adjusting the mixture screw. If it makes no difference or if it is far away from the 1 1/2 turns out, you may have a blockage in the air passage. Clymers has a picture of the carb showing this opening. Idle mixture is in play for more than just idle.
Bing book says the 45 is correct and needle should be in position 2. Carb cleaner sprayed in the intake side with both the idle jet and the screw removed opened mine. Also had a broken O ring on the idle jet earlier. O rings from a Bing kit broke while those bought from a dealer went on easily and appear to be good. Main and needle jets can vary with different carbs on the R100, but if the left is good, the right should also be good with the same jets and under the same conditions. When both sides act the same then you can start changing needle from 2 to 3 and then playing with jets but making changes to stock settings before finding the reason for rich running on one side is going to waste a lot of time. Bobs sells a kit of just o rings and gaskets for rebuild or sells individual parts for carbs. Shipping may cost more than the cost of an o ring. If there is a dealer nearby, they may have or can order needed parts.

Bob

Make sure the choke/enrichners are all the way off and replace the base gasket & O ring in the choke assembly. These are their own separate carbs and if not working properly ( no air leaks etc ) then they will leak fuel or vacuum. It is common for the screws to loosen and the base gasket to be pulled in causing problems. Set the cables so the choke/enrichner is all the way off with about 1 mm of slack in the cable.

The next would be the mixture screw. I've replaced O rings and then ended up just replacing the whole mixture screw, which worked much better. Some turn these screws in too hard and damage the tip.

Periodically everything rubber, and all internal parts ( floats included ) should be replaced inside the carbs and that would depend on when this kind of things was done on a 33 yrs old bike. If possible weigh your floats. New ones are in the area of 12.2 grams, so if yours are heavier suspect that they are sinking as they are filling up with fuel.

Thank you thank you. I had the identical problem with both carbs running grossly rich and performance suffering. Mileage dropped down to about 30mpg. I was in despair and getting ready for something major. Followed your advice and presto! The jets were all crudded up like yours. What a difference a few specks of dirt can make. R100GSPD Dick

Followed your advice and presto! The jets were all crudded up like yours. What a difference a few specks of dirt can make. R100GSPD Dick

EXACTLY ! Because a carb works on a system run by vacuum. There is no "pressure" to blow even tiny amounts of crud out of the way. And unless you use a carb cleaner, like StarTron fuel treatment, any restriction inside the jets caused by a buildup of varnish will make your bike's power feel anemic.

Often times, this buildup happens so slowly that you don't really notice it, until you go to pass another vehicle on the open road one day and there's the shock of discovery... there's simply not enough power !

:ohmy: