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From a posting by me, edited here, from the Airheads Mailing LIST...slightly edited here..

I had an off-LIST inquiry about some things, and in the original query, the Airhead was complaining considerably, about certain parts of the airhead design. He had a nice list of 'bad design items'. BMW is not perfect, witness the transmission circlip problems, clutch lever clips, valve seats, etc. But, one of his complaints was about the throttle at the handlebars, with its requirement for cleaning and re-lubrication every few years, and its complexity, including the need to synchronize the marks on the cam gear and throttle sleeve. The BMW airhead throttle design is far better, NOT WORSE!, than many others. Whether it is the single-cable type (I prefer that one slightly), or the dual-cable type. The cam gear was designed to be NON-linear. That is, as the throttle is moved a certain amount, the cam moves slowly, and as the throttle is much more rotated, as when much more power is needed, the cam moves faster, pulling the cable faster/more for any particular number of degrees of throttle movement. The advantage is that as one takes off gently from a stop, there is more and smoother control over the carburetors. Further, the design is such that the throttle cable is a straight-pull, and there is no bending of the inner cable, as in many other types of designs, and the throttle cable at the bars-end can be expected to last a very long time, without fraying or breakage. If you clean and relubricate the parts, they will wear very slowly. There is some complication involved if you ever have to replace the gears, they are not the same from early and late models, and it is a crap-shoot with some dealership parts department, if you will get the right ones, so bring along the old ones. I think...???.,....that Tom Cutter addressed this parts problem some long time ago on this LIST. (see later in this article) Do not tie the throttle cables down with wire-ties/wire-wraps where they were not supposed to be. Do not route cables wrongly, including with sharp bends. Don't lubricate them, although DO lubricate the end barrels (for SURE at the carburetor ends, after every bike bath). If you have the type of throttle on the bars with ONE cable coming out of it, you have (unless you are riding a BMW single!) a tubular junction under the tank...it has an adjustment...for the upper cable free play. Do NOT bend the cable at the left carburetor when checking your oil. There is no need for that oil dipstick to be overly tightened. Bending the left cable is a prime cause for that left cable to have increased friction, possibly spread some coils on the wrapped sheath (making that carb fun and games to synchronize, if bad enough), and eventually break an inner strand...usually where you can see it between the throttle lever on the carb, and the sheath. A single strand found broken (You DO inspect these cables regularly, don't you?), will usually result in a total cable failure in a few hundred miles or so. In your on-bike tools and parts, you should have a spare clutch cable, and a spare throttle cable. Besides the clutch cable, I carry only a lower throttle cable on my single-upper cable airheads. There are all sorts of lengths of throttle cables, be sure you have the correct one for your model, your carburetors, and your type of bars.

Throttle gears and cams:
This has caused a LOT of confusing over the years, as BMW has changed the design of some of the parts (especially for the SINGLE throttle cable versions) in the twist-grip throttle assembly, and some earlier parts are NO LONGER AVAILABLE. BMW also modified the top cover and gasket of the on-bars rectangular brake master cylinder, information on that is in the brakes article. BMW in its infinite wisdom, does NOT list the throttle assembly in the carburetion section of Katalogs....no, it is in Section 32, which is STEERING. The throttle cam is not the same for 32 and 40 mm carburetors. Many original cam gears are NLA. The proper cover plate to use with the later cam assembly is 32-72-1-457-050. The cover plate, 32-72-1-242-561 UNDERSIDE, was flatter at the screw hole. The newer plate has a round protrusion at the center, and that is about 1/2" diameter and maybe 1/4" deep.
The throttle cover was 32-72-1-233-538 for the earliest models.
The new cams are thinner, don't fit the old assembly cover.
The cam assembly for the 40 mm carburetors is now 32-72-1-457-081.
The cam assembly for the 32 mm carburetors is now 32-72-1-457-080.
The throttle TUBE did NOT change.
The cam gear for the R65 series is 32-72-1-238-378.

Be SURE what you purchase will fit!! This throttle assembly has been a PIA, and it may be best for you to contact knowledgeable Airhead folks, such as Ted Porter's Beemershop, if you don't understand what is what.