R75 (1941) misfiring-any ideas?

[Anthony Evans]

My R75 (1943) misses the occasional firing at high RPM. It is quiet misfiring-you need to listen. Starts well, fine at idle. Choke is off as soon as the bike is started, mixture seems fine. Bike can also seem to lack a little power at lower revs (no misfiring at low revs)and occasionally feels like the engine might die if I don't keep the revs up. I suspect ignition, maybe points etc. but any bright ideas?

[Allan Atherton]

It would only take a minute to check the point gap and the timing.

[Prosper Keating]

You could also check the condensor. If it is failing, you will see bright sparking at the ignition points. Not an occasional spark or a soft glimmer from time to time but constant sparking. Another possibility is the spark plugs. You may be using the recommended plugs or their equivalent but bear in mind the changes in petrol in the last few years. Also, if the plugs you are using have integral suppressors, this could affect ignition as you describe, especially if your magneto is a little on the weak side.

PK