To inspect or service the ignition system, it is easiest to remove the whole magneto assembly from the engine. Disconnect the H-T lead from the spark plug. Loosen the set screw which is behind the armature back plate and which clamps the back plate on to the engine’s crank housing.
The back plate is also clamped to the crank housing with a support bracket. On some Villiers ignition system, there is an alternative clamping arrangement allowing some rotation of the back plate so as to “tune” the ignition timing.
With a bit of back-and-forth rotation of the back plate around the axis of the crankshaft, the entire armature back plate should come away from the engine (see image) and can be overhauled more easily on a well-lit and clean workbench (to avoid losing small but crucial parts).