What is the reason why the engine speed can't come down after the motorcycle loosens the throttle?

If the carburetor cable and oil needle are normal, it is a "out of control" phenomenon. The best way is to change the carburetor.

When the valve clearance becomes smaller, the valve is not tightly closed or the intake pipe leaks slightly, it may happen that the throttle speed slowly returns to normal. Whether the intake pipe leaks oil is not determined by whether it leaks oil, but by disassembling and checking whether the sealing ring and plane are normal.

In this case, clean and adjust the carburetor first, and then find other reasons. When the air humidity is high, it may affect the quality of the mixture, which is more likely to appear in rainy and snowy weather or after washing the car. Whether there is a problem with the throttle and the oil needle can be judged by observing whether the throttle can be quickly lowered to the bottom and whether the oil needle card is installed correctly.

If this happens after moving the carburetor, it is probably still a carburetor problem.