Or the tiniest smear of grease on the cam. I mean a drop, smeared over the cam and more-or-less wiped off until there is nothing to see. Redo every ten years, unless you're desperate for something to do. Not sure which magneto you have - ring cam or face cam - but the fibre rubbing block does wear a bit over the years. If it's badly worn you can probably find a new one.
Leon