Being my first post on this message board, I just purchased an '02 Kona NuNu and it needs some work. Primarily, the rear derailleur will not shift between 1st and 4th, only 5-9.

Standard advice on old Shimano shifters is to buy a can of spray WD-40 and squirt it into the shifter body (you'll want to partially squeeze the brake lever to expose the innards). Once you've flushed the internals of the shifter out, spray in some gray lithium grease.

At that age, it's probably a good idea to replace the cables and housings. Lube everything that can be lubed (derailer pivots, brake pivots, etc.). And your brake pads are probably hard, replacement pads will stop much better.

If it only goes through cogs 5-9, but the shifter goes 1-9, it indicates the cable is too long.
https://www.parktool.com/blog/repair...&area%5B%5D=52

My apologies for not being as detailed. It will shift down from 9 to 5, but after that, it stops indexing. With the bike being 18 years old, new shifter's may be in order.
As I just recently acquired the bike, along with 2 others, though, I would prefer to fix what is not functioning so that the family can enjoy riding together. Especially since the weather that we are experiencing is perfect right now.

Thank you PD, I took your advice and flushed the shifter out with Ballistol and then began to run it up and down through the index. Works like a charm!