Are you talking about the idle screw next to the coolant reserve tank? It does exactly what it's named... It controls the idle speed of the engine, to manually speed it up or slow it down accordingly. I tend to keep mine around 1,000 rpm. The idle speed on your bike may change with variations in the fuel/air mixture entering the engine caused by fluctuatiing atmospheric temperature and humidity levels. More simply put, your idle will need adjusting between hot & humid to cool & dry conditions.