If you think it might be related to carb icing you could duct hot air from the outside of the exaust manifold to the air cleaner for the wintertime.
I think I would swap carbs just to verify the problem.
Also, in my experience, very few few filters are completely full, with no air pocket, when sitting. I think it is because they usually sit horizontal and don't bleed out the air properly, I also think that is why most filters aren't clear (so you can't see this).
Tim
