I have an XL 125 that has a fresh top end, new coil and condenser, points and plug. I have checked the timing advancer and it is O.K. It starts and idles fine but when you give it some gas to rev it up it sputters and pops and won't rev. I have cleaned the carb, tried different jetting, float settings, but the problem persists. Can a bad stator allow it to start but not rev? It does not have a battery at this time- does this bike need a battery to run properly?