I had the same problem with some old Suntour friction shifters. They were severely worn so it was not a design or install problem. In frustration I just took them apart and reassembled them using Elmers School Glue. Wow... They held up for a year and a half before I had to do it again...
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No matter where you're at... There you are... Δf:=f(1/2)-f(-1/2)