I have recently put a 450r carb in my 400ex... but now i have a issue that i can not seem to get a answer for... i've gotten lots of ideas...

When i start it cold... starts right up... idles great... runs great.. no problems.... but if i let it run for a bit to warm up the idle climbs... and once it's up i get not get it down... idle stop screw has not affect... mixture screw does not help... i have no air leaks... atleast none i can find.... and orhter then the high idle when warm.. i have no other problems... anyone have any ideas????