There's a TON of information available if you do a web search on fuel mixture.bnutz24 said:
The biggest problem with running too lean is excessive cylinder temperature potentially causing valve or piston damage.
The easiest way to get some idea is to closely inspect the spark plug tips after they've been used for a while.
If the tips are black you are running too rich and building up carbon deposits.
If white, you are too lean.
The ceramic at the tip should be a light brown color and the tip should not be burned or fouled.