How does a colder air temperature affect an engine's tune?

It's all about the fuel/air ratio so when colder/denser air enters the carb, I think more fuel is needed to maintain that ratio. So the main(high end)needle would need to be adjusted fatter not leaner to compensate.

Is that reasoning correct or am i backwards?

you actually need to richen your motor up because the air is so cold and thin theres less air entering the engine. I think thats how it goes. Been over 2 years since I had to tune a motor. Im definatly sure you need to richen it tho.

Cold air is DENSER. More Oxygen entering the motor, you're going to need to fatten it up quite a bit, or you will run lean. basicly you're going to need to completly retune the motor for the weather, if you've not kept adjusting it between warmer and colder days you might require somehwat drastic adjustments.

Yeah, you need to richen it up for colder weather.

Wow, i know i should be getting less runtime but yesterday when it was in the negatives im pretty sure it was at least 13-14 minutes, didint time it tho.

like I said, when I run in cold I get like 1-2 minutes less when tuned for racing.

I never let my motors go lean.