To sum up: I recently bought an '83 R100RS with about 50K miles on it, from the original owner. The bike has good service records from a local BMW dealer. I rode the bike for about a month, it seemed strong and in good tune, but something changed while I was on a 100 mile ride a few weeks ago.
I've tried troubleshooting a few things, but the bike is still missing noticeably in 5th gear at around 4K RPM. The bike doesn't lose power, it just feels like the choke is on and it's running too rich. Both spark plugs still look clean and white, no cracks in porcelain, gapped okay. I don't feel it as much in lower gears, at lower RPMs. It still feels "off", just not as much at lower speeds around town.
I've swapped carbs with a set from another bike, no improvement. Checked choke and throttle cables, they seem okay.
I've checked spark plug leads for failure and correct resistance, swapped out the ignition coils, checked to see if the bean can was loose, no improvement. Only thing I haven't tried is to adjust timing, but it seems to me that if the timing was off, the bike would run poorly in all gears, at all RPMs.
Thanks for any additional suggestions and thoughts.