I've had a few magnetos through the workshop during the past couple of months where a simple service, easily performed by the owner, would have fixed the problem. The vast majority of us know that, if you're riding round on a bike with a magneto containing the original capacitor, you should also be riding around with your fingers crossed (metaphorically, of course). The symptom of capacitor failure is 'difficult to start when hot' or 'impossible to start when hot'. This is because the insulation in the capacitor is weakened by heat so, when hot, the capacitor shorts out. The older the capacitor the more 'bridges' across the conductive plates will have been caused by repeated heating and cooling. The original capacitors weren't perfectly airtight either so they absorb moisture from the air which adds to their woes.
A similar condition is caused by an oil contaminated earth brush. If you think back to your school days and, for those who are familiar with electrical circuits anyway, electricity has to run in a circuit. Any interruption will stop the flow and whatever it is supposed to be driving will stop. In a magneto, the earth brush plays a vital role in ensuring that the voltage generated in the magneto armature finds its way back home to the magneto armature. The circuit (simplified) is as follows:- armature (HT coil), slip ring, pickup brush, brush spring, pickup, HT lead, plug cap, plug electrode, plug gap, plug body, cylinder head, engine, magneto body, earth brush holder, earth brush spring, earth brush, armature. Magnetos work best at high revs so, at kickstart speed, a bike will be, by definition, harder to start. If the earth brush is oily it may be effective in prevented the limited voltage tracking back to earth, whereas at high revs, the increased voltage may be able to fight through the oil on the earth brush.
It's obviously a very simple task to clean the earth brush. Just unscrew it from the top of your magneto, give it a wash in white spirit or carb cleaner and, while you've got it out, check the continuity between the brush and its 'pod' with a multimeter - should be no more than a few ohms (5-10 max). If the earth brush is oily you will also need to clean the earth track of the magneto by poking a cotton bud or similar down the earth brush pod hole in the mag body, take the spark plugs out and kick the bike over a few times. Do this until the cotton bud is clean. Maybe the oil seal is becoming ineffective or maybe, because you haven't done this for the past ten years (!! sorry to be rude !!) a bit of grease has migrated onto the earth track.
....'Darling ! I'm just nipping into the garage for a minute!
