Perculation or boiling the gas in the carb in the middle of december is very unlikely. It is also not that common of an issue. I have only seen two cars that had that issue in my life of hotrodding. Cant imagine that is the problem here since the engone is cold and wont stay running without the choke for 20 minutes.
More of a problem with super high compression engines that have to idle for a long time. Usally a little air under the hood cleans them right up.
That being said spacer wont hurt.