Cub Cadet backfire / won't start

To sum up, bought it new last season, used 4 times, left the gas in (because I'm a dummy). Sunday it won't start. Emptied old gas, pulled / cleaned carb put in a new spark plug tonight.
It started, ran for 2 mins, made a big pop, shot a few quick flames out the exhaust and died. Now when you pull the cord, it pops with a single flame and won't start. Now it's beyond my knowledge.