I have a 1992 K100 RS. It runs good at an idle and good up to 3,000 RPM. From 4,000 to 5,000 rpm it surges very noticeably. After 5000 it runs smooth again. Here is what I have done so far.
Replaced all plugs and two wires (the wires were vibrating off over time)
Replaced all vacuum caps
Checked for air leaks, and checked vacuum hoses
Synced the throttle bodies
Good fresh fuel
Valves were checked a few months (and very few miles ago) by Max BMW
I am thinking it might be a weak coil, does that sound right?
Thanks for you thoughts!