HIF6 carbs have an internal temp compensator attached to the jet assembly.

As nearly as I can tell the biggest difference is not air density (because
it is the weight of air flowing past the jet that pulls in the fuel, not just
the volume) but in ease of vaporization. That is one reason why winter gas
contains butane, summer gas does not. But any gas vaporizes (and thus forms
a flammable mixture) more easily when it is warm than when it is cold. The
colder it is the lower proportion of the gas is vaporized and thus the harder
it is to make it fire. Note for example that "Starting Fluids" dry pretty
quickly when you spray them - they are more volatile and thus make a combustible
mixture more easily. In starting (when you don't have much turbulence in the
air path to assist mixing and vaporization) you enrich the mixture so that
even though a low percentage of the fuel vaporizes, you have a low percentage
of a smaller quantity and thus more vaporized fuel that will ignite.
You've probably noticed that you also have a "fast idle" feature with the choke.
This is to keep the flow (and turbulence) up so that you don't have to have
quite such a rich mixture to keep the engine going until it gets warm.

So I guess the overall answer is that it is probably good practice, if you know
what you are doing, to enrich the mixture a bit in cold weather and lean it out
when it gets warm. However you can probably beat this (especially with the
cast arn combined manifold) by just pulling the choke a little further out and
keeping it out a little longer in the winter. This is a more natural function
on the HS6 and earlier carbs, whose choke moves the main jet, than on the HIF6,
which has a separate rich mixture mechanism.

Have I muddied the water enough, or should I write more......
--
George Downs Bartlesville, Heart of the USA!