I recently installed my new K&N air filter [after cleaning my Twin-Air and deciding I'd had enough] on my '00 YZ426F. Never been so dirty as when "cleaning" a foam air filter! Anyway... I installed the K&N and I don't know if it's just wishfull thinking on my part or not... but the bike just seems much more responsive. All that fresh air must be doing something good for it. Still haven't done the BK-mod [need to clear a space in the garage to work properly]. But at the moment... the bike is running great! Anyone else have good/bad luck with the K&N?