Just so you guy's know the correct terminology is Afterfire if it's out the exhaust, Backfire if it's coming out of the carb.
If it's coming out the exhaust, I think it could be an exhaust valve or it's way to rich with poor timing. Consider this: the valve would have to be at least slightly open to ignite the air/fuel and blow out the tail. The timing could be way too retarded (firing way too late). Easy fix!