The coil has been changed. There is a video with sound in first post.When you say a backfire into the carb, is it more like a popping noise than a gunshot noise? If so, it maybe a faulty coil feeding the spark plugs. Another symptom is it'll run fine at higher revs.
Isn't it now all controlled by the engine management unit?
Bad ignition timing
Bad valve timing
Inlet valve not sealing correctly
I think Jim has it, ignition usually doesn't pop back through the carb.