The idea was to use the most capable chassis that was ever used on the Airheads to make an über-RS. The R100R has the Paralever rear suspension and the 41mm showa forks, giving potential for a great handling package. The execution is not straightforward, though.
There are plenty of problems. The things that make the R100R good are also unique in ways that confound the ability to make an RS. And some of the characteristics are limitations without really being benefits at all.
The GS/R exhaust is unique in that the headers neck down to 36mm and maintain that all the way into the center muffler. From there you have a left-side single muffler, due to interference problems with the paralever and the brake lever. So, to have a muffler that fits, you need a left-only muffler, which requires that collector box, which means you have to stay with 36mm headers. A dual exhaust ALMOST fits, if you can deal with the brake lever and sidestand. Probably an aftermarket 2-into-1 could be made to work, possibly with a different brake lever.
Bottom line is that custom fabrication will be needed to duplicate the RS dual 40mm exhaust.
The late GS/R have the K switchgear, requiring longer exposed cable length due to the cable holder at the throttle housing. The 40mm carbs also require extra exposed length due to the longer throttle arms, and BMW only sold one length of the ultra-long exposed cable. That was to fit the Euro R100R/GS, and the overall cable length is pretty long. There is no BMW cable with the ultra-long exposed length that isn't very long overall. My solution will be to modify the control housing to accept dual cables, so I can use shorter cables from earlier 40mm bikes.
The RS always had a stamped upper plate, whereas the R has a taller cast one. Handlebar fitment is an issue; with the stock RS handlebar the grips can't be rotated to the correct angle because the brake hose and the clutch cable strike the triple clamp.
A true RS needs a 2.91 final drive, and the Paralever driveshaft arrangement (it's a unique driveshaft; the only Airhead driveshaft with modern spline at the rear) only has proper gearsets available in 3.09 and 3.20. The problem is that the input spline is 12mm longer on the Airhead Paralever than it is on the Oilheads and K-bikes, since the driveshaft U-joints are about 12mm forward of the swingarm pivots. One alternative is to have a 2.91 pinion spline lengthened; another is to move the driveshaft back either with a spacer or by adapting an Oilhead driveshaft.
9.5:1 pistons are installed with dual-plug heads. At this point I am still using the original 42/40mm valves. BMW's 40-to-32 adapter stubs connect to 40mm Bing carbs. See this page for details on 40mm conversions.
The ignition system is the Silent Hektik Powerblock system. This completely replaces the stock system, eliminating the canister and ignition controller. Instead, there is a timing pickup on the alternator nose and a separate controller unit with 16 programs from which to choose. There is a mounting plate available for the dual coils but this was not sent with the kit I ordered; a friend fabricated a bracket from a bit of square tubing.
So far, the result is an engine that really has some guts. There isn't a huge amount of grunt off the line, but there's a ton of midrange power compared to the stock GS/R configuration.R100RS+