Sound like the engine is choking for air. A couple quick possibilities: 1 - The choke is partially closed, and 2 - Something has plugged the air cleaner.
Pull the air inlet hose off the carburetor and check the choke plate - it should be wide open when the choke knob is pushed in. Then with the air hose still disconnected start the engine. If it throttles up OK now there's a restriction somewhere in the air cleaner.