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. 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, which is increasing the size jet inside the carburator, thereby increasing the amount of fuel the engine can pull from the carburator, 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 attatched to a slide valve inside the carburator the moves when the throttle is twisted, 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 richly. Conversly, if you "lower" the needle setting, there will be less fuel available and the engine will run leaner.