400EX??? you can try jetting it for the cold the 400's are extremely tempermental when it come to the cold weather, anyone up here with them will agree.
What I recomend is to simple set the choke to half pump the throttle 4 or 5 good pumps and barly I mean BARELY hold the throttle and start it when it starts keep that throttle position for about 2 or 3 min gradually turning off the choke after about 5 or so min you should be able to ride.