Ive recently had similar problems, if yours has a waster spark coil, take both spark plugs out and attach the HT leads to them, sit the plugs on the exhaust or something that will give a good earth. (they both need to be earthed)
then with the dizzy cap off and with the engine in a position so as the points are closed, ignition on, flick the pints open by hand and watch that both plugs spark together. flick it a number of times to be sure that on every occasion they both give a strong simultaneous spark if they dont try them in different positions, swap leads, if the same plug plays up renew it.
ive never done the timing on a bis, but the manuals describe it simply enough. on the air cooled its easiest to remove the rocker cover, turn the engine until both valves on cylinder 1 (nearest to you) are closed (i.e spring fully released/raised) and that the two valves on cylinder 2 are level. the notch on the crank pully should be in line with the timing marks. Line the notch up with the marker that indicates 10 degrees before tdc. then release the dizzy clamp nut so that you can turn the dizzy, twist the dizzy so that the points are just about to open and job done.
check that the rocker cover isnt cranked down so hard that its holding the rockers down.
