I am fixing up a Trek 260 Mountain Track. Its about 15 years old, and has a triple chainring that is spot welded or something similar (as opposed to chainring bolts). Bottom line is that you can't take it apart. Rear is 6 speed. It was shifting poorly, I finally realized what the problem is: Spacing between chainrings seems more than normal. The chain actually drops between the teeth and gets stuck there before it gets a chance to properly engage on them. My theory is that the original chain(not on the bike now) might have been wider. I tried a new SRAM 6-7-8 chain, that was no help. Adjustments, lubrication, no change. Sometimes shifting occurs OK, but usually not.
Hope someone can help. The bike is for sale, and needs to function properly.
Thank you everyone,