Just want to mention, I have worked on a few Helicomatic freewheels with success and have the needed specialized tools to do so. I even took an old Helicomatic hub shell and modified it to work as my "vise" tool for removing sprockets for cleaning and inspection.

I agree with verktyg it does also sound as if that one sprocket is worn badly. It can be difficult to detect this while the sprocket is mounted because the wear occurs worse on the back side.

BTW, I recently serviced a 5 speed Regina Corse Oro sent to me and IIRC it was 13-23. The 18T center sprocket was one of the most worn sprockets I've ever seen on a freewheel. Reminded me of some "shark fin" teeth I've seen on really worn out chainrings. The interesting thing was the other sprockets showed little to no wear and when I polished them they were close to NOS.


Front side of a severely worn Regina Oro sprocket

Back side of a severely worn Regina Oro sprocket