5. When reading the spark plug, the porcelain insulator that surrounds the center electrode should be malt-brown in color. If this insulator is white or a very light brown, the mixture is too lean. If the insulator is black and oily, the mixture is too rich.
6. If the spark plug showed rich or lean symptoms on the plug check, refer to the part of Chapter 1 that covers the carburetor being checked and make the necessary changes that are recommended in the section that pertains to the particular throttle opening being checked.
7. As in the rough tuning section, better results will be obtained if the same tuning sequence is adhered to. That is, check the lowest throttle opening first, and then proceed to the next largest opening and so on, finishing at the largest.
8. When fine tuning a carburetor, it is sometimes recommended to do a second series of plug readings to insure that any adjustments or changes made in the first series have not changed the mixture proportions at other throttle openings.