Is the choke on or off? Let it warm up with only as much choke as you need. If the bike will not stay running after it's warm and without the choke, then it could be a simple pilot jet cleaning will fix that. Also check the air mixture screw. Start at about 1.5 turns out from all the way in. If the bike stalls, try turning it 1/4 turn inward for richer condition. Then check the needle clip height. If it was jetted for summer riding and it's now winter, I would drop the clip down the needle to the next setting. ie, go from #2 to #3 for richer condition.
Basically the colder the weather, then the bike must be adjusted to richer settings to be able to run well. In the summer, you will have completely different settings than winter. pilot jet will prob stay the same but needle position, main jet and air screw will all change.
