I am thinking this is similar to my 4300 and there is a friction disk (washers) that you can tighten to have the throttle stay at one location. Someone may have oiled them at some point in time not knowing there is an adjustment.
Go to your third pic (apparently 3 pics with duplicates ?? ) and you will see the mechanism that has the double nuts on the backside to adjust (and it appears someone has greased it which may be the problem and some gas to cut that grease may be in order).
If you go to jdparts.com, you can put in the 950 model number, find the 850, 950, and 1050 compact utility on the list, then go to alphanumeric listing to find the throttle you are talking about.