Phil,
I'm not sure that I agree with your rationale.
Denser air means a greater pressure drop in the carb = MORE gas sucked in
through the jet. Actually the density change is pretty small compared to
other factors since each degree drop only increases the density by the
inverse of the absolute (Kelvin or rankine) temperature, so the difference
between 20°C (293K) and 0°C (273°K) is only 293 divided by 273 or an increase
of about 7%.
The majority of the reason for enriching the mixture is
that cold gasoline and cold air lead to much poorer vaporization of the fuel
into the air and thus more difficulty in getting it to make a flammable mixture.
Gasoline is made more volatile in the winter to offset this somewhat. They
usually do this by blending butane and other more volatile hydrocarbons into
the gasoline.
--
George Downs Bartlesville, Heart of the USA!