My friend just bought a 2006 CRF450X and put a Pro Circuit TI-4 full exhaust system on it. He tells me it runs fine, but after a few runs now we pulled the spark plug to find its slightly on the lean side. Not very much though. Is anyone else running the same system? Can anyone give some thoughts as to the correct jetting? I was thinking of just raising the needle a position, going up one main jet size, and maybe a slight turn on the mixture screw. Any thoughts? I should add that we're about 100ft above sea level and we ride in 50 to 90 degree weather.