NEED HELP! Backfiring!

Thanks for all the responses guys! It isn't happening at low or high rpm, only upon quick snap of throttle. If you Rev it up real slow, it will be fine but will get some popping out of the exhaust. The cam was definitely broken in properly (done by a well known shop in Canada called Beatty and Woods). Triple checked to make sure plug wires weren't crossed. I'm thinking now a plug MAY be fouling, but im not sure. If one plug is bad or going bad/weak, could it cause this issue?