Some of you may be familiar with the serie of vids, Marco, a member of the German forum, has posted on the dissassembly (total!) and reassembly of his CB500 carbs. I believe there are 7 vids or so in total.
BTW, it's with hesitation that I mention them: personally I do NOT recommend to undertake what he has done. I myself have never felt the need to even separate the carbs from one another. But that's not the subject here.
At 18:20 in the vid below he shows, he has added small O-rings around the airscrews.
Originally that O-ring is
not there. His reasoning: to avoid any risk of
Falschluft. I don't see the danger; I have no O-rings there.
This is just one of his vids, the start (part 1) of the reassembly: