[Juergen]

There should not be a gap between the points at idle. The contacts are used to close the circuit so the electromagnet pulls the diaphram down. The missing nut then pulls the lower points bridge down, opening the circuit which then moves the diaphram and the point bridge up to close the circuit again. Then the cycle continues. The second nut locks the first nut in place. The time which the points are closed under power is one factor in the tone.

You need to test with a high amperage source of power (like a 6 volt battery). Ground the horn base and apply power to the lead. Move the big nut down until you get the proper vibrating frequency (good tone). Then lock the nut with the second nut. If the diaphram does not move with power and the points closed, clean the points and try again. If the coil still does not pull, apply power to the lower points (bypassing the points) and see if the coil works. If not you have a coil/circuit problem.