darrell's answer is the simple one. the stoichiometric(i think that is how you spell it) fuel mix is 14.7:1. when you add fuel, that ratio goes down. the best power air/fuel ratio seems to be about 13:1. when you remove fuel the ratio goes up. the best economy a/f mix seems to be 16:1. anything under 14.7:1 is considered rich, and anything over 14.7:1 is considered lean. by the way the ratio indicates the amount of air to the amount of fuel, and it can be expressed in many ways. usually in pounds IE:
14.7 pounds of air to 1 pound of fuel.