I'm not sure what or where the coasting enricher is or what it does. If there were air leaks on the manifold or exhaust flanges wouldn't the bike behave poorly all the time?
Vacuum leaks related to the flange area on the manifold, generally manifest themselves during those times when the vacuum is at it's highest inside the manifold. This happens when the throttle is closed. When you close the throttle, the throttle plate in the back of the carb
throat(it is behind the slide you see when you look in the carb throat) closes off the throat except for a small area. The vacuum goes up as the carb switches from the venturi back to simple vacuum from the motor, to supply fuel.
If the leak is bad enough, yes, it will affect the motors peformance all the the time. It would become unride-able.
Slight leaks can be a nightmare to find. People will often spend more time fiddling and looking, then they would just removing, surfacing the flanges, and replacing the manifold.
Roadies have a reputation for manifold leaks. I'd put it right at the top of things I'd do to a new to me Roadstar I had purchased, as far as getting the bike in good running condition. Especially, if I was chasing a gremlin. As mentioned pull the enricher slightly, the next time the bike seems to want to die on a hard decel
. If the bike stays running, then you know the enricher is supplying the needed fuel to counter the extra air from the leak.