Hello

I recently bought a new 2012 kx250f. It has about 20 rides on it and has been serviced regularly. The last ride I noticed that it was starting to have trouble idling. After that ride I took the tank off and changed the plug, changed the oil and put a new air filter in it.
The next day I tried to start it and it would not start. (over 100 kicks). I pulled the bike on 4th gear and it started up and idled with the choke on. Once the bike got warm it runs good when you are on the throttle but as soon as you let off it dies. Any ideas???? Please help Thanks