It is generally understood that current piston engines are fueled lean to meet emission requirements. In the boxer engine, this can be troublesome causing detonation, higher combustion chamber temperatures, driveability issues and reduced engine output. Later boxers employ knock sensors to detect and offset the issue of detonation.
In our attempts to improve performance, many boxer owners install free flowing exhaust systems, high flow air filters and other gadgets in an attempt to make more noise and more power.
Upon doing so, we see questions and comments such as yours regarding the appearance of lean fueling conditions. And while few boxer owners report engine damage, there seems to be an increase of the installation of aftermarket pipes and air cleaners. These pipes and air cleaners promote more engine air flow but do nothing to increase fuel flow. If you insist on aftermarket pipes and air cleaners, be prepared to spend more on electronics to increase fuel delivery along with the increased air entering your engine.
Over Christmas, my son trailered his Honda V-twin here to Alabama. One morning it would not cold start. Removal of the air cleaner showed why. The oil from the K&N had settled in the carburetor and choked off the pilot jet as well as left deposits on the carburetor slide. A spray with carb cleaner got things running again. But, it was evident from a plug check that the A/F mixture was way off.
If you insist on aftermarket air cleaners, be sure they provide effective cleaning of the air entering the engine. Some studies show these aftermarket air filters pass considerable dirt compared to the OE paper air filter. You have been warned.