It's probably running too lean. As the weather gets colder, the air gets more dense. That means there's more oxygen in the same volume of air, but the fuel stays the same, so you end up with a higher air/fuel ratio than normal. Either rejet it for the colder weather and get it running right, or it's best to just keep it in the garage until the weather gets warm again and it starts running right. Running too lean causes more heat than normal, and can damage your engine.