Backfiring, sounds like a timing belt jump or a unadjusted distributor, Also could be a bad valve, but bad timing cause valves to open or close to early or too late cause fuel to explode while a valve is still open and that's when a backfire occurs. Your hearing the time of combustion explosion when the the valve isn't sealed and it explodes down the exhaust manifold instead. carb back fires is the same except the explosion leaks out an intake valve up and out the carb. Usually burning eyebrows in the process.