Fuel/air mixture refers to the balance of air to fuel that enters the cylinder in a moped's engine and is ignited by a spark plug during operation. The optimum ratio, known as the stoichiometric ratio, occurs when you have 14.7 times the mass of air to fuel. Sometimes a moped will not run optimally if this mixture is too rich (too much fuel/not enough air) or too lean (too little fuel/too much air). The mixture can be made richer at WOT by upjetting, thereby increasing the amount of fuel the engine can pull from the carburetor, or restricting airflow which is usually done by changing the air filter setup. The fuel/air mixture at all other throttle positions can be changed via needle adjustment. The needle, which is attached to a slide valve inside, controls how much gas can get to the engine when the throttle is anywhere other than wide open. If you "raise" the needle setting, you are going to allow more fuel to the engine which will make it run more rich. Conversely, if you "lower" the needle setting, there will be less fuel available and the engine will run more lean.