A couple things you might check. Take the 2 screws out and turn the breather cover out of the way. With the engine running about half throttle, take a hold of the brass clevis and push down and pull up and see how it responds. Down should lower the speed and up should raise the speed, and you should feel resistance both ways. Then, with the engine stopped, take the pin out of the clevis and move the throttle shaft lever full range,up and down, and it should work freely. It could be that the tang that fits into the carb throttle shaft slot is badly worn or rusted that could cause the symptoms your seeing , also could be rust and dirt inside the throttle shaft tube. I've seen where the tang is rusted to where their is only a thread left.
Then take the side cover off and check to see if everything is getting oiled and check for excessive wear. Not very common ,but the spring could be stretched with gaps between the coils. Hope this helps.
Thanks Charlieu and Red Turbo. Bob