I turned on the carburetor and turned the throttle handle, but it didn't work.

When unscrewing the upper cover of carburetor or other parts, the throttle gets stuck and cannot fall down, resulting in high engine speed. Check whether the idle screw is tightened too tightly. If the upper cover of carburetor moves, just remove and reinstall the throttle. The throttle valve is directional. Note that there is a slot on the throttle valve, which can only fall to the bottom when a pin is installed on the carburetor.