You need to know what carby it is, then just google the stock settings for that carby. From the factory the stock settings are always rich
Also all carby's are different and have different stock settings, plus the temperature, humidity, and even altitude affect the settings, so there is no such thing as the "correct" or "best" setting
It could also be the clutch slipping or not engaging correctly. The stock Rovan clutches are the worst of the worst