After riding with my buddy for a few hours the other day i noticed that when he revved it real hard there would be a slight bit of smoke coming out of his exhaust. His bike is a strong running 2005 yz450f. We took a look at it closer before we loaded it up to head home. It only happens on a good quick rev.Its not blue smoke so i dont think its burning oil. The smoke has more of a white/gray tint to it.. Anyone have any ideas?? Thanks to all.