Why does my Murray riding mower do this?

I have a 1997 Murray with a Briggs 12.5hp I/C. When I turn it off after about an hour, every now and then it will shoot out a large flame about three feet long; not a common split second backfire flame, this looks like something from a flamethrower. It makes a quiet sound that kinda sounds like it's whispering "pow". Then it lets out this cloud of very light blue smoke, not quite blue enough to be pure oil blow-by. It shoots it out full blast for about 4 seconds, then about 7 seconds of it pouring out, then it lightens up and stops. It starts right back up after this happens. All I know is that it burns about a gallon of oil an hour. It runs very strong, like a Briggs should, except it backfires a lot and it burns oil terribly. I offroad with it (hills, wheelies, rock climbing, etc), and I beat the living **** out of it.