Some more info on your jetting would be good, ie what pilot,main size is in it and the needle setting,you say it ran fine in the summer ,well it's winter know and for every drop of 20 degrees you should be going up on your mains one size,a good indication of being lean is back firing.
Just a helpfull hint here,when you change your jetting make sure the choke tube goes back in place and the carb slides are in the correct way(cut out of slide facing towards the airbox)
Your lean don't run it until it's jetted correct.
DON'T PUT SMALLER JETS IN HER