Had the same problem with my '02 Softtail until today. In temp at around 40 or colder, my plugs would foul and she stuttered and stalled. Enrichener (choke) was all the way in (open). Had my wife sew up a carb sock today from nylon leggings. First day out this year, with temps in the 30's. Ran perfect. Pulled the sock off on the way home and plugs fouled. Cleaned up plug and have the sock back on till the temps warm up.

 

No one has mentioned if the carbs are stock. The air/fuel is lean from the factory, in cold weather the fuel does not evaporate as well, and the mixture leans a bit more. Also, the higher density with cold air leans the mixture too. Perhaps a little choke will help.

 

So I finally figured out what the issue was with my cold weather carb problem. In our remote community, our local gas is just not up to par, if not totally contaminated. I brought some gas back with me from Newfoundland this summer. Since my last fill up in the Harley caused nothing but sputtering, I decided to drain the tank and put in the gas I had leftover from outside town. Sure enough, even at 32F this morning, the bike ran just fine. Not a happy camper now knowing that we don't have suitable gas here to even run my bike correctly. Bummer. I'm down to a 1/4 tank with what seems to be plenty of good riding weather left before the snow flies. Octane Booster and Seafaom didn't seem to help the last tank of bad gas.

 

In certain geographic regions there is summer and winter blended gasoline. If you get a tankful of the wrong blend during a seasonal change over you can experience drivability issues. There is also regional boutique blends mandated for emissions control.
It seems your bike is sensitive to either one. Have you checked for intake seal leaks as suggested previously? Perhaps experimenting with carb jetting to achieve a balance between ambient air temperatures will help.

 

I have sprayed around the intake to check for leaks. I think experimenting with jetting may have to be my next approach. Sure is nice to have it running so nice, though, on just plain old gas.