I hope you didn't have the same problem I just found. Backfiring exactly as you mention but only the left side. Tried plug swapping, pulled one wire at a time and put a plug in it and each fired so wires were good. Never backfired before so I knew no carb issue. Looked at Dist cap and rotor all looked good and again each plug wire fired. Took off my valve covers and cranked the engine with the coil wire off and viola!! Rocker arm barely moving. Flat lobe on my hydraulic flat tappet cam.