I nearly missed your posting because it wasn't in the Yanmar sub-section here.
In the manual found on this site and at the Yanmar Tractor Owners Group, we see the layout of your shifter.
View attachment 79433
And the other lever.
View attachment 79434
In the first image, remove the shifter knob. Unscrew it. Remove the cover plate. Now there is some access to inspect parts for rust and binding. Look at item #13. Is it rusted, worn (no spring life left), not moving like it should? Item 10 is shown in 3 segments in the diagram. It's one piece. Ensure it's lubricated well. Did the shifter rod (item #10) wear thing item #21 the big washer?
Inspect item #2. Looks like a travel linkage of some type. My shifter is setup different on the floor. So you are needing to examine things closer. IF all looks good at this first level, the casted housing needs to be removed to go the next level deeper. The gasket in the first image shows 4 bolts, yet only 2 are shown pictorially. Examine and note the sequence of each bolt length and hardware under the head.
When your are deeper, examine the second spring, item #6 if you can. There is a single wear ball bearing seated with the spring. I bet it fits in a pocket to the cover casting.
The diagram does not show how item #8 fits with item #3. Or if either should.
The second image may be the clutch shifter for the desired operation range of the PowerShifter to work in. I show it here because these items are linked together into the transmission gearing.
There are 2 more levers I do not understand without the knowledge of operating a YM1610. Those would be the next 2 diagrams in the parts manual.
Typically, Yanmar and Deere often times mention to use GL-4 grease. Do this for items above the casting cover. GL-4 should not be used in areas of hydraulic fluid (J20C). No cross contamination allowed.
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