It's also common for lean conditions to cause the backfire to be out the carb(s) instead of the tailpipe.
Always check electrical things first before adjusting the carb(s). Start with a general inspection of the ignition components... plugs, plug wires, dizzy cap and rotor, coil connection.... Once you've replaced any suspect parts dress and reset the points (preferably setting dwell, not gap), then set the ignition timing. With the engine cool, set the valves clearance. If these mechanical and electric settings don't address your problem, then it will be time to consider adjusting the carbs.