Cold air is leaner, but just a tiny little bit. Because of the cold, the air is denser and more air can go through the carb while the amount of gas mixed through it wil stay the same. Some high-performance supercharged cars have something called an intercooler, this cools the air just so the engine can take in more air, the cars carburettors are adjusted to perform propperly with the cooler air. Also, when the engine of your bike is still cold, the air will not be warmed up by the carburettor as it passes through and the mixture will be much too lean, that's why the carb has a choke, so you can block the airflow patially to make up for the larger amount of air coming through the carb. As the engine warms up the carburettor, the carburettor will warm up the air passing through, so when you're driving in cold weather the mixture will be a little leaner but not much. This is also the answer to why engines have trouble starting in cold weather.
RE: cold air = lean?
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