Backfire from exhaust

Backfiring through the exhaust could be from a broken insulator on a sparkplug. How do I know??? My brother had a 340 Challenger that backfired through the exhaust. Long story short, he changed the timing chain, retimed it, carb checked, ran through a check on everything, still couldn't find the problem. Never changed the sparkplugs because they were new. Found one with a cracked insulator that was misfiring. $2.00 (or $200.00) later, problem solved!!! Just sayin'!!! Geof