We are working on a 78 that backfires through the carb. The carb has been off and completely rebuilt, all the smog and vacuum devices have been cleaned or replaced, the compression is great, the valves have been adjusted several times, and the cam timing appears to be correct, cap and rotor, etc new. Any suggestions? It seemed to be a little better with all the smog stuff disconnected, but the car is going to have to be "smogged" so it has to be on and functioning.