That is a symptom of an air (vacuum) leak, probably at the carburetor where it attaches to the intake manifold, or, the manifold to the cylinder. Those are the most common places to start looking.
With the engine idling, spray a mist of water around those areas. The idle speed will decrease if the engine is drawing in air.
We always suggest that the carb be sealed to the intake and the intake manifold gasket be replaced with a good quality gasket material instead of the kit supplied one.
There are other areas to look at but start there. Also make sure your throttle/cable is working smoothly and there is no binding in any parts. Cable should not be kinked or bent sharply or tied too tightly to the frame. The slide, or barrel in the carb must travel smoothly and fully up and down when you twist the throttle grip. You can remove the air cleaner and watch the slide as you twist the throttle. It should close nearly completely with only a small gap at the bottom in the idle position.
Good luck.
Tom