Just bought this Vespa 100 Sport small frame:

The bike has a ducati flywheel and CDI installed by a local shop, no battery, so hardwired. The horn works intermittently. I removed the horn to make sure it was plugged in, it looks fine. Sometimes it works and is really loud. I don't have a socket to get off the nut on the front of the horn, but I tried messing with that screw on the back, but I can't tell if it's an adjuster or what.
Does anyone know about these horns and whether they need to be cleaned or something every so often?
Thank you!!