Horn only works while driving

[Marshall V. Daut]

The horn needs oiling in its two felt pads. Also, back the adjusting screw out one or two clicks to allow the armature to spin easier. Readjust after oiling.

The reason the horn only works when driving is because your generator is putting out more juice at speed than when idling. The extra juice is enough to overcome a dry armature shaft to make the horn's armature spin. This is indicative of an armature that needs oiling and perhaps the commutator plates sanded with 220 wet-dry sandpaper.
Marshall