The carb on the Vespa I used to have was crap, it really was the most worthless carburettor I have ever seen. Old Honda's (like my own) are equipped with Keihin carburettors, you can run with them for as long as you want. Keihin carburettors are a lot better then a lot of people think, they want a 27mm Mikuni flatslide or a Dell'orto, and when they've got money they want a Takegawa, Daytona or Kitaco high performance carb, but Keihin belongs right among those three high quality carbs.
I've got a 20mm Keihin carburettor on my Honda SS50, it's originally the carb for a Honda CB125. It runs great, even when it's freezing, and it delivers a lot of power too.