Not sure I understand your last comment about valve settings. Your problem may indeed be the timing but we can not do ignition timing if the valves are not right first.
The BMW Airhead has two cylinders with two pistons going up to TDC (Top Dead Center) alternately. When the piston on the left is at TDC on the compression stroke the piston on the right is at TDC on the exhaust stroke. Only one set of valves can be set at a time. The side at TDC on compression is the one to set. Then the engine is turned over through 360* till the other side is at TDC on the compression stroke.
For setting valves the flywheel is watched through the timing window for the OT mark. If the valves are close to correct when the OT mark appears one side will have both valves loose. This is the side to set. The side to set is the side that the intake valve has just closed on. The other side will have valves not loose, at least not both of them.
The procedure for setting the valves is in all the manuals. You should have a manual.
What settings, size feeler gauges, are you using? I use .006" intake and .008" exhaust. In metric these are .15 and .20.