This is the small, cone-shaped jet at the bottom of the main jet stack. They come in various sizes ranging 82-165. Most vintage Vespas will use one in the 85-116 range, though performance-modded engines can drink a lot more gas and use much bigger jets. The larger the number, the more fuel will flow through.
Jetting is something that takes trial and error. See our jetting blog for more info.
