So I'm having a very strange problem. I have an old 10 speed Chimo with friction shifters on the stem. A few months back I noticed that when I downshift to the inner chainring, my front derailleur can't shift it back up to the outer chainring. I put it off and only used 5 gears till I tried to fix it today. I flipped the bike upside down and played around with it and it shifted perfectly back and forth. I then righted the bike and had someone hold up the back tire. It also shifted just fine. But when I rode it, once again it refused to shift back up to the outer chainring. Any ideas what this could possibly be? Could my weight while riding be doing something that I'm not noticing? It's driving me bonkers!
The extra pressure on the chain when you're pedaling can make it harder for the chain to shift over, even when it shifts well holding the bike up.
A couple things you should check:
1. alignment of the front derailleur. The outside of the cage should be parallel to the chainrings and the the derailleur should sit 2-3 mm above the top of the big chainring.
2. Outer limit screw. There are inner and outer limit screws on the derailleur that stop the derailleur from moving too far in or out. You might need to back the outer screw out a little to let the derailleur move over farther and push the chain up to the outer ring. Too far and it will let the chain fall off on the outside. You have to find the happy medium.
3. tighen up the cable. If the shifter cable has a lot of slack, it may not be able to move the derailleur far enough over to shift. With the derailleur on the small chainring, the cable should have no slack, but not be super tight.
Intermittent shifter problems can also be caused by a frayed cable or kinks in the housing. Check that (or replace it anyways, cable and housing are cheap).