My couple cents' worth: I have had terrible luck with the foam filter media in any form on these bikes. They are made for limited-range RPM engines, like chainsaws, lawn mowers, stuff like that. When airflow starts moving quickly thru them, they "suck closed" and begin to choke off the air. In some bikes (like notably BWM and Harley Twins) this works in their favor, as making the low-speed throttle responsive in those critters means leaving out the parts needed for richness at near-nosebleed RPMs - which those engine hate...so in those select environs, the foam works. Not so much in engines with a 10+:1 RPM turndown ratio, like SOHC4 or Katana bikes. Those who may notice the 'successful' applications of foam filters see them in bikes like the Kawi Fours (7500 RPM redlines), without realizing WHY they still work there: those bikes re dead-flat lean mixers above 6500 RPM, to help protect the Kawi warranty.
If you want to REALLY see the foam filters' epic FAIL, try them on a CB450/500 Twin...they not only won't run well, they won't even start when hot, for over-richness during low-speed "filter closure", and no longer need a choke for cold-morning starts. And, they won't rev past 5000 RPM, either...and those engines are 12:1 turndown RPM ratio!
