At this link is a table of cassette sprocket spacing
http://sheldonbrown.com/k7.html useful when mixing rear drivetrain indexing components from different manufacturers.
For Suntour 7 speed, the spacing between sprockets is listed as
3 x 2.8 mm(L)
3 x 3.0 mm(H)
Question: Does '3 x 2.8 mm(L)' refer to the three smallest diameter sprockets or the three largest diameter sprockets?
My Suntour Accushift 7 speed shifters index well mated to a Shimano HG 7 speed cassette and Deore LX rear derailleur. Not perfectly, but very well. From the table at the above link, I noticed that if I use three eight speed spacers (width=3.0mm) in place of several seven speed spacers (width=3.15mm) I could hit the 4.8mm c-c sprocket width almost exactly--within 0.05mm.
It wasn't easy, but I found some eight speed spacers locally. I had guessed that 3 x 2.8 mm(L) meant the three largest diameter sprockets. When that didn't improve things, I tried the 8 speed spacers between the smallest sprockets. Same, no progress from vg before the changes to pure perfection.
Is it that the incremental improvement from vg to perfection given this set of components is just not possible?