The suntour may index to a suntour pull which had a different cassette spacing on some of the cogs. To be honest, your best bet would be to

A) ensure your cassette/freewheel is shimano standard spacing. If it is a suntour cassette you may have issues
B) either find index shifters for suntour or shimano depending on the cassette or replace the rear derailleur with a shimano unit. If you can't find a decent 7 speed rear derailleur for less than $20 new or $10 used I'd be surprised.