3ph lever not returning:
Going by memory as I don't have a WSM at work.
There is a rod that runs from the lever assembly back to the right side of the HST that connects to the upper 3ph arm. The rod has two springs, one on either side of the bracket that attaches to the 3ph arm and there are two lock nuts that can be screwed in/out to adj the spring tension. This spring tension is what "returns" the 3ph lever to neutral. You can access this assembly in the gap between the top of the right rear wheel and the fender.
Your best bet is to shine a light in this area and work the 3ph lever to see how it works. You shoul dbe able to adj accordingly.
I was having this type of problem on my
BX2200 until I cleaned this area of rod and springs and then lubed the rod with a spray lubricant so that the rod slides freely - worked like new afterwards.