China's classic car stalled at 4000 rpm. Why?

The neutral refueling speed drops slowly. Carefully clean the carburetor first, check whether the mixing ratio of the carburetor is wrong, check whether there is slight air leakage between the carburetor and the intake pipe between the carburetor and the engine, whether the valve clearance is too small or the valve is improperly closed, whether the throttle valve is stuck, and whether the gasoline is not good. If it is a new car, the valve may not be closed properly.