The bike idles rough and stalls if I let go the throttle lever while warming up even if the choke is full on. Also, exhaust backfires when slowing down. Acceleration is not smooth either as it used to be. I have only about 400 miles on it. Any idea what to check? Thanks.

 

first I would try to turn the idle up a litttle and see if it helps.

 

when you pull your spark plugs what color are the tips? Black & carbed up, wet, etc...
have you replaced exhaust changed to a K&N Filter, or any other changes to the performance?
Pour a bottle of seafoam in the gas tank and take it for a ride also, if you drove it slow/idled to much it may be fouling plugs. seafoam cleans the crud out very well.