It sounds like the linkage is stuck or the governor is out of adjustment. Remove the fuel tank, check linkage for binding or the governor spring broken/unhooked.
If necessary, adjust the governor by loosening the governor arm at the governor shaft that comes out of the engine. Move the governor arm away from the carb as far as it goes, and turn the governor shaft counter-clockwise as far as it will go. While holding these in that position, tighten the governor arm to the shaft again.