The real question: does it backfire under part load below around 3000 rpm? Or does it smooth out under full load above 3000? If both are true, you've got at least one plugged idle jet, and maybe need a cleaning, synch, and mixture adjustment.
If it backfires no matter what load or rpm, look at spark.
Before you do any changes, wait until it's ICE cold, and adjust the valves. After you adjust the valves, do a compression check, all cylinders should be within 10%. If that's true, move on to spark. Check or change the plugs, and check out points, distributor cap, and wires.
You need three things for an engine to run. Mechanical, spark, then fuel. In that order