I had the same thing happen to me. Found out that the end of the throttle spring came loose from the plastic cable linkage that slides back forth above your air cleaner box.
FuzzyDriver posted a link to a youtube video about this issue on my thread about no throttle response, just 2 threads down below yours. The video is a dura force but the throttles are set up just
the same as your F engine 10515. I don't know what else it could be. Let us know if you get it figured out.
I don’t know what nut you are talking about. Are you sure that the air vane is not hitting something, preventing it from closing the throttle? With the engine not running you should be about to move it very, very freely.