listen up kiddies before your spark plug you have to have a proper CDI like the jaguar CDI . There is none better I have found. The ngk B8hs the Gold colored one is a hot plug . And as far as mpg goes . if your looking to go fast well do not expect any other outcome except going fast burns more gas.
oh just an FYI b8hs is not a hot plug. 8 series is the coldest you can get from NGK. and a stock CDI is most often not the reason for vibration. cranks are notorious for not being balanced to the rod and piston the motor is later assembled with,, then there is a whole rash of bevel gears not being really centered. all contributes