Just rebuilt vespa. new loom, switches lights fitted. Electrics are AC. All lights and indicators are working fine until I press the horn and all lights go off. When lights are not on I have to rev the scoot up to get the horn to sound. What's going wrong.?
It would be extremely useful if you would tell us more details about your Vespa, as different types exist (which have different electrical systems).
You might have a Vespa with battery or one without battery.
You might have a vespa with a DC horn or with an AC horn, but we don't have a cristal ball...