Going home for the weekend one Friday evening I had just joined the A604 in Cambridge when the power died, the cause seeming to be slipped ignition timing. I removed the mag chain cover and loosened and refitted the tapered sprocket to reset the timing and set off again only to have it recur almost immediately. A second attempt proved no better so I left the bike at a friendly garage and hitched home.
Returning after duty on Monday I used some fine grinding paste to achieve a good finish on the taper fitment and then drove back to camp where it failed again as I drove up to the radio section where it was kept.
I removed the magneto and started to strip it down. On pulling the armature from the body the taper end stayed in place in the drive end bearing. The broken shaft end had the familiar ‘S’ of a shear break.
Did I really drive almost seven miles on a broken magneto shaft? Was the inherent magnetic field around the shaft helping to hold it together well enough to produce drive?
One for Richard or Roy to puzzle over – or anyone else with similar experience.
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