It's a little known fact that the Bosch electronic ignition unit fitted as standard to twins from 09/1980 onwards incorporates a mechanical advance unit, with bob weights and springs. Not available separately from BMW, these springs stretch over time, and can contribute significantly to poor running.
Replace those old springs; improve the performance of your engine! Sold individually, there are two springs in each unit.
Please note: these are not the same as the springs in the points canister used before 09/1980.