You're probably running headers and big pipes, which allows enough oxygen to re-ignite exhaust gases. No problem though. First check your timing...it could be retarded, and needs to be advanced. Then check your air/fuel mix. You're probably rich (are the insides of your tailpipes dark or black?). If these two fixes don't cure the backfiring, you could be a candidate for a valve job. I'd get it fixed though...ignition in the exhaust system will shorten the life of your pipes. Good luck!