I've never found any difference in performance with my Rossi 45 or other engines but of course there must be some loss because it's a slight restriction. In some applications they're used as a fine tuning aid in that the fine filter gives the same affect on fuel draw as reducing the venturi size by about .020" and about .010" when using the coarse filter. Another advantage is that they trap any fuel that tries to spit back out the carb so your engine tends to stay cleaner on the outside.