Been lurking for a while here but just finally signed up. Great forum. So I got this 2003 YZ450F. Bike runs awesome for the first 4 miles on the trail, then in a short time bogs down, and becomes unresponsive, then finally stalls. It seems to be heat related. When it happens, I tried pulling in the hot start, and then trying the choke to see if it was fuel related - no change either way. It seems to idle normal-ish, but when the throttle is cracked, and it is in gear, only the tone changes. Totally no power, and there is nothing I can do to keep it from dying. Once it cools down for a bit (20 minutesit starts and runs great again for another couple miles until it get good and hot. For what it's worth it was slow trail riding so I'm s the bike got pretty hot, but it never boiled over. Main jet is a 175, riding pretty close to sea level with a full length exhaust and header. Supposedly has a brand new CDI box replaced by previous owner (now I know why he put it in). Does anyone know if they have a temp sensor that kills the ignition partially if it gets too hot? I would love to check it all out, but riding 4 miles then ripping the seat, shrouds, and gas tank off in the middle of nowhere (before it cools off) to check things out is a little inconvenient. I am leaning away from thinking it is a fuel/plug problem, because it comes on suddenly, not like a fouled plug where is gets sluggish at first. Response is very snappy, up until it hits, then all of a sudden it seems to be firing only every-other and will build no RPM what so ever, then another 100 yards or so and it stalls out completely. Would love to hear from anyone with something similar or maybe point me in a direction here. Feel free to be technical - I am an ASE Master Tech and own my own repair shop for my day job. Thanks in advance.