Hi Andrew,
For info without getting into too much detail - a magneto has an armature with 2 coil windings - one with heavy gauge wire, and one with fine wire. Thick winding is low voltage and the fine wire is high voltage and connected to the plugs.
Inside the body of the armature is a capacitor/condenser which assists with creating a "fat" spark.
The armature is connected to the points and to the spark plugs through a rotating connection called the slip ring.
The main body of the magneto houses a permanent magnet - the energy from this is what generates the spark.
The permanent magnet fits on top of a shaped pole piece which concentrates the magnetic flux at the appropriate point for the firing point of the magneto.
In the case of V twins like ours, the pole piece is specially shaped to achieve spark at 42 degrees. But there is a compromise as the flux "density" will always be better for one spark plug than the other. Hence a good working magneto with points set to full retard will often only spark weakly or not at all on one plug until the points are moved back towards full advance.
NOW ... Considering the above -
With the magneto on the bike - Check the slip ring - these get contaminated with carbon from the brushes and can form a continual track thus bypassing the plugs. Also check the brushes.
Check, and carefully clean the points.
If there's no spark after this, then it would be worth removing the magneto and checking the turning resistance when rotated. There should be definite points of resistance where the points open to generate the spark. If there is little or no perceptible resistance, then it's likely the permanent magnet requires re-magnetising - so you'll need to take it to the repairers anyway.
The original capacitor/condenser fitted into these old magnetos rarely fail unless someone has fitted a newer inferior part - this requires a rebuild to replace.
Finally - the fine wire on the armature was traditionally hand wound and then taped over the outside with cloth and lacquer. The coil windings are basically loose wound and are susceptible to break. Therefore when the bike (and magneto) are hot, the windings expand and the break cannot be bridged by the electrical current. When it has cooled down, everything contracts and it runs for a while then stops again when hot.
New rebuilds often include resin vacuum impregnation of the rebuilt armature to stop this happening.
Hope that gives you some things to consider.
Good luck with it.
Regards Mick
