I dont understand this. One why someone would tell you back firing is a piston/ ring issue. I belive its more carb/jet issue.
But anyways. I would get a new cable if it seems something is holding it back, there really is only one good reason for that
and cables do need to be replaced i do mine every year. But im sticking with the other's sounds like a air leak, to make it rev. then
Want to shut down when you let go of the gas. Aslo check the carbs. Take off the air filter, and use one hand to open the throttle up slowly,
and make sure the slides are moving at the same time and the same distance. I would beat money your problem lies here not piston and rings.