the bike: 88 schwinn premis
the drive train: shimano RX100
brief history: i built this bike a while back using rx100 f&r derailleur and crank. in the back i was using a 7 speed cassette that was spaced out on a 9 speed cassette body. i turned the bike into a single speed for a while and now i'm gearing it back up using the same parts as above. the only difference is i'm using a different cassette, still a 7 speed though.

The problem i'm having is my chain line is way off. when i have the front on the small chain wheel, the chain grinds on the big chain wheel. its only straight if i have it in the 4th gear up. iv changed cassettes on plenty of bikes with no problem, i don't understand why i'm having this problem