Do your carbs mount with those vibration isolating things? The O ring seals?

If so, then go & buy a couple of new ones, then pop the carbs off one at a time. These O rings do have a finite life span & it sounds like yours might be sucking air. If your O rings are NOT in poor condition & sucking air, then keep the new ones for later as the old ones will eventually deteriorate.

As I said, it sounds like it's sucking air somewhere & these rings are a relatively common cause.