chain line problem
 

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

Are you by any chance running a chain which is too wide? Is it possible that you have left an un-needed spacer behind the cassette?

If needed you can easily put a 2 mm bottom bracket spacer between the BB driveside cup and the frame. This will move the crankset out 2 mm.

Changing the cassette should not change the chainline. Perhaps you have not stacked the cogs and spacers the same as before.

Start by finding what the actual chainline is. You can do this empirically by measuring from the chainrings to the downtube and adding half the diameter of the tube. Then compare to the standards or to the cassettes chainline (distance from inside of dropout, subtracted from the dropout width).

You can also measure it directly by placing a straightedge (edgewise, not flat) on a secant against the outer chainring, and carrying it back to the cassette, then allowing for the offset to the center of the chainrings seeing where it compares to the center of the cassette.

Based on that, you can decide if you're best option is to move the cranks outboard, check rear triangle centering, increase chainring separation, or use a narrower chain.