Originally Posted by
Doohickie
A better upgrade would be to put a
freewheel made for indexed shifting on the bike. It has ramps that aid shifting, improving shifting performance even with a friction shifter. It gives performance almost as good as indexed without having to worry about whether the throws of the indexed shifter match your derailleur.
Not all index freewheels have those ramps, but Shimano ones do, and it definitely helps with friction shifting.
I'm sure there's something about indexing and Suntour systems on Sheldon Brown's website. It may be of use to you here, but I don't have time to find it right now.
Edit: I found this, which might be of use
http://www.sheldonbrown.com/upgrade-gears.html