I once had an AC1 with the same complaint.
I removed the crankcase breather hose and fitting, lifted the flat rubber disc, and beneath that is a fitting with multiple holes. I screwed this out using a snap ring pliers inserted into the holes and turned.
Looking through the hole I could see the top of the arm that moves the rack, and as I moved the Stop/Run lever back and forth I could tell that the pivot rod for the arm was bent slightly in a way that prevented the rack from closing fully.
The pivot rod is quite soft metal, and I simply took a screwdriver and hammer and Carefully and Gently tapped the end of the rod in the direction that it needed to go and that solved the problem.