hi i am bent wrench. i live in south central mt. i have an 05 660 grizzly. just rebuilt the top end new valves and all. the machine starts normal, idles fine,takes throttle to about half, then falls off misfires under load,back fires through exhaust. plug is sooty as if rich. i have tried leaner jetting,no improvement,i also replaced the diaphragms,float,needle and seat. i have tried different clip positions,fuel is fresh,looking for other possibilities.it will rev. in neutral is this a stater issue if so can i run a test . i have a digital and a pocket tester