When it's colder out, you may need to richen the mix. The cooler temps increase the air density. So relative to April, you're putting more air in your mix even if you don't change the needles. This will lean the mixture and could cause the engine to flame out easily. So I'd richen the HSN a bit and it should be fine. The biggest thing is to make sure you can get the engine up to a safe operating temperature (+200*).