74 Valiant 318 Backfire when hot

The fact that it doesn't do it cold but does it hot eliminate a few things (kind of strange to). Could be as simple as when it warms up, a leak occurs, leaning it out and it's getting the "lean snap". Maybe when its' fully warm and idling you could spray carb cleaner around the carb base/vacuum hoses/and intake. If it revs up a little, your on to something.