/6 manifold

Intake or exhaust ?
back fire would be caused by being out of time, bad distributor cap, timing chain wore out and jumped a tooth, Vacuum leak ... how often does it back fire ? and is it just doing it when you throttle up ?
If your intake runners have obstruction (dirt dobber nest) you will have a cylinder not getting fuel and not firing at all and that plug would be hit and miss and would be gas fowled.
is this a daily driver and did it just started doing this ?
Need a little more info on this.