Pre war magnetos with iron magnets in the flywheel can and do lose their magnetic strength over time. Post war magnetos with ALNICO magnets seldom if ever have this problem. If you are having weak spark and everything in the mag checks out OK, then your flywheel magnets may need recharging. Placing a keeper ( a nail or piece of scrap steel) across the magnets poles whenever the flywheel is removed will help prevent the magnets from draining.
Here’s a short video showing how it’s done.
https://youtu.be/QnkAEhaxK9I
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This reply was modified 4 years, 7 months ago by
Mumbles.