It just started backfiring out the exhaust. I checked the timing, changed plugs and wires, changed dist. cap. Took the valve cover off and checked the cam It didn't wipe any lobes. Also changed coil. It does have petronix ignition starting to wonder if it burned the petronix up. I'm lost don't know what to check next REALLY NEED SOME HELP . 340 solid lift cam THANKS!