[rich weyand]

One other complication to note. The number of oxygen molecules in a given volume of air varies with temperature. So when it gets cold out, the mixture numbers will run leaner, and when hot, they will run richer.

I have the thermac air cleaner on my truck, so the inlet temperature is always about the same, winter or summer. BUT, when I get on it, the vacuum drop will open the air door and pull in ambient air. So I run my secondaries with an A/FR of about 12.5 in 70 degree F air. In the winter, that number will be much higher when the air door opens, but I don't want to have to swap needles twice a year.

Fuel injection compensates for all of this stuff on the fly. This is just a carb consideration, particularly since you aren't running the thermac air cleaner. Going into the winter, lower numbers now would be better, because they will climb as the temperature falls.