Main Jet Gurtner

You should always start off with a too-large main jet when going about jetting your carburetor; then decrease jet size in steps of 5 until your engine is running fine. Run your scooter with open throttle for a short time and then cut the engine dead, remove the spark plug and take a look at it. If it is black or/and overly oily, the mixture is too rich and you'll have to decrease jet size. If it is whitish, the mixture is too lean, which can lead to engine seizure! A proper mixture is indicated by a medium brownish/reddish spark plug.