I've got a 2008 Hisun 500 cc and I've been having problems with it cutting out under load or at a certain speed ,or when I floor it its always the same noise and no power I've changed spark plug ,coil coil wire valves, ,timing chain ,filter throttle cable ,carburetor , stator ,,,,,so this started one day while riding and it backfired a bunch of times and it shut off going down the road but came back on before we stopped we made it home and its never been the same since....please help