i have been chasing a occasional flutter while mowing for quite some time. i have a 190cc 6hp briggs flathead.could a partially sheared flywheel key cause this intermittent miss?mower is 10years old and i have NEVER struck any object while mowing so i dont know why the key could be sheared or partially sheared.i am a fanatic about fresh no ethanol stabilised gas ,clean air filter,etc.if i would visually inspect the key would i be able to see if it is faulty? thank you for any input!