hi! guys.i need a help for my 250cc bandit.problem is spark plug die quickly.placed the original japan plugs cr8e ,carburetor clean restored and tuned for company settings.but plugs die after few miles running.but i know a fault the charging is not working correctly.idle position 13.8v-14v but when i accelerate from 1000 to up.charging going down from 14v to13.2,12.8v like that.is that the reason for die the spark plug.and i saw the crack of 1 magnet on fly wheel. i attached the photo here,please guys help me.give me an correct solution