It's sucking air from somewhere. That is the only way a gasoline engine can RPM. You have an air leak somewhere. The problem is finding it.

On my daughters Breeze the carb choke was sticking and it was sucking air from a passage in back of the carb by the carb throat. When the choke is on it will uncover a passage that bypasses the slide. Make sure you choke is free and working. The only way to do that is take the plunger out and see if it's free.