I done a couple of small mods today on my 09 YZ. First thing I will say is the bike came very lean from the factory, to the point it was getting annoying trying to start it. Mods today with just under 3 hours on the bike: No-toil screenless filter system R&D acpump with adjustable leak jet, and spring 48 pilot jet. WOW, I had been thinking of going with a larger rear sprocket to get more bottom end to equal the Honda, but no longer needed. These small and simple mods awoke the bottom end greatly. No hesitation at all, very crisp throttle response. IMO it was money well spent.