Spaco SI Carburetor Main Jets

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.