Ok......
Is the battery up?
Is the charging system charging?
Does it spit and pop back through the carb when its backfiring?
Does it get better or worst by choking it?
It sounds like the pilot jet or circuit in the carb has something in it.
From 0 -1/4 throttle is the pilot jet..........make sure the jets and circuits are clean............
If its not the carb..............it could be the ignition coil being bad or not getting enough amps from the battery/charging system.
Also there could be a vacuum leak somewhere...........check all the hoses for cuts, breaks, or not being on correctly.