Reading spark plugs is the best method to measure proper main jet selection. Proper detection of correct jetting keeps potential problems from occurring well in advance of the damaging results that could take place. Reading plugs should only be done by doing a "Throttle Chop" (a hard wide open throttle run followed immediately and simultaneously by killing the ignition, pulling in the clutch, and closing the throttle, coasting to a stop and pulling the plugs out for inspection).

The plug on the left indicates an overly lean condition, although it looks brand new. Further symptoms would include slight specs appearing on the surfaces indicating pre-ignition. The center plug shows near perfect running condition of a light ash coloring on the porcelain which is a normal result of burning gasoline. The plug on the right shows excessive oil burning (black gritty surface) and overly rich fuel mixture (black, smooth shiny surfaces).