Idle Jet (55/160); SI Carbs
So a 45/120 jet would run richer than a 45/140, because the 120 allows less air to pass. To make things more complicated, some vintage Vespas came with "plugged" idle jets, which have no air hole. These are available in sizes 42 and 50.
Jetting is something that takes trial and error. See our wiki page of for more info.