Pull the carburetor and clean it. This failing, On most regular motorcycles there is a manual petcock that is used to shut fuel off to the carburetor(s). On most scooters the fuel !@#$ is vacuum operated. As the engine goes through its intake cycle, there is a vacuum line from the intake that goes to the fuel !@#$ that controls flow. If the engine is not running the fuel does not flow. If the vacuum lines have a kink in them or are loose or cracked they will cause the engine to run lean and also prevent correct fuel flow to the carburetor, especially at high speed.