06 YZ450, have owned it for a little over 3 years, has run great until recently. I first noticed something amiss about a month ago when partway though a ride it wanted to stall out at idle. Next thing I noticed was a lot of decel popping, so I played around with the fuel screw but couldn't make it go away. Next thing I noticed was a cracked head pipe. Replaced the head pipe, but it still wasn't running right and was blowing white smoke out the crankcase breather tube. Thing figured out it was losing coolant, so tore the top end down. Everything looked OK but the head bolts had loosened way up and was likely allowing coolant to leak into the cylinder and burn off, hence the white smoke. So we replaced the head and base gaskets, replaced the rings since we were there already, and did a valve adjustment. The valves weren't too far out, and the cylinder and piston looked good. When it is cold, it starts up on the first kick now, but as soon as the bike gets warmed up it starts to run like crap. If you keep the throttle opened up above idle it won't die, but as soon as it returns to idle it dies. I have played around with the fuel screw and it doesn't fix the problem. I am about 1.75 turns out on the screw, which is where I think I always was in years past. Any thoughts?