EDIT: When I pull on the derailer cable, bypassing the index shifter, things work just fine -- so it has to be a mis-indexing issue. I thought I had bought a friction shifter for the front! Oh well....

Hey all,

I'm running into trouble with the front derailer on my bike. I've got 3 chainrings up front; when I shift from largest/outside to smallest/inside, things work. But when I shift outward from the smallest ring, I can't reach the middle ring. Instead, I get a lot of grinding noises as the chain rubs against the ring but can't quite lift onto the teeth. Eventually, it will usually switch to the largest chainring, skipping the middle one. Or it may "lock" on the teeth of the largest ring.

Some specifics, if they help:

• 1983 Trek 620
• Shimano derailer, can't see an indication of the model. It's clearly made for a front triple. Most of the drivetrain components seem to be from the early to mid 1990s.
• Biopace chain-rings, 28-36-48 (Yes, the weird oval ones, and yes, they were there when I bought the bike)
• SRAM MRX twist shifter (the new part of the ensemble). FWIW, the full range of shifting occurs between the markings "1" and "2" on the indicator.

I can't say for sure how well it worked before I changed the shifter from the thumb-lever I had when I bought the bike used, because I started swapping things out after only a short test ride. My bad....

Thanks!