just a thought, you don't have the chock on

 

maybe I will have to check that out on mine 2000 400 that went in for broken spider gears came out wouldnt run worth crap, the new mechanic had forgot to set valves, had shims in the wrong place on the high low selector, had rebuilt the carb and wasnt asked to, and also took the rings off and cleaned the grooves and put same rings back on. The bike would race at idle, when got warm would die and not start. I adjusted valves and that took care of that, but would still foul plugs,or should I say they would just quit firing after about 10 hrs. I have went through the carb several times and found nothing, so maybe I will check that.

 

sounds like the chock or enricher is stuck open

 

My '06 400 would do the same thing, but only when I rode to higher elevations. Was running rich and had to change the jet. Still runs a little rich at higher altitudes but all I have to do is remove the airbox cover and it runs a lot better. Might consider 'air-jetting' the airbox with a couple of hoses that I could block off or open up as needed.