It can and should be done while the engine is not running so it doesnt suck up any dirt in the air. The throttle trim should be at zero before making the adjustments. After setting the trim to zero turn on the transmitter, then the reciever, now turn the idle screw clockwise until the carb opening is at about 1-1.5mm. Once you do that check to be sure that there is a small opening (about .5mm) when you apply the brakes, and you should be good to go![8D]