I serviced the K&N air filter at the same time. I had been wanting to check the valve adjustment since I have read a lot of debates here and elsewhere on the filtration of the K & N. I am now satisfied that it does a very good job of filtering harmful media from the intake air. If a lot of contaminates had been passing through, the intake valves would have been TIGHT. The dirt wears the valve seat and face. I have seen this a lot in motorcycles. I worked in a Honda motorcycle dealership service department four years mainly on dirt bikes.