Streetgrinder

I have a 1985 22r, which I've made a lot of progress with by unscrambling the emissions vacuum lines to where they were supposed to be. In the underhood diagram there is the regular VSV in bold lines, and some kind of auxiliary VSV in dotted lines. This dotted line circuit attaches to a vacuum switch at the passenger's side base of the carb, and to one of the outlets of the carb fuel filter with a T to the air cleaner. The dotted line VSV has a little air filter and an inlet which is making a noticable sucking sound all the time. This seems odd to me. Anyone have a clue to what this extra circuit is all about? I doubt the part is available.