Moisture do a range check of both your rear derailleur and your rear shifter. These old systems were designed for up to 6 speeds for friction shifters. I'll bet the rear derailleur can be adjusted for the full range of that 9 speed cassette. Your shift lever is another story. It was designed for 5 or 6 speeds and may not have enough travel to accommodate 9 speeds. A lot of these shifters had bent tabs to limit travel. One can straighten the tabs out and travel by them to get more shifter travel. I had to do that on an old Suntour friction MTB thumb shifter setup. The Suntour was then able to handle a modern 10 speed cassette with modern Shimano Shadow MTB derailleur.
The right hand Suntour friction shifter had its travel limit tabs bent straight. Shifter now handles a modern 10 cassette and MTB derailleur. These old Suntour MTB shifters are really good.