2009 YZ450F, about 50 hours on it. After getting the engine to a nice warm temp (20 min of riding or so) my bike starts to run like it's lean (pops and has more engine braking) and stallsl a lot. I does this with predictable repetition, and it seems to be related to engine temp. I've tried different jetting, which seems to make a difference in the way it runs for the first 20 min, but it does not improve the running weird and stalling issue after 20 min. I just checked the valves, and they are all on the tight side. All of the intake valves will barely fit a .10mm and all the exhaust will barely fit a .20mm. They fit, but with a bit of pushing, definitely more than what I would call a light drag. I think each of them would nearly come in to the middle of the range with a .05mm, but maybe I'll get some Honda shims so I can get them in .025mm increments. So are these symptoms of tight valves?