I'm having a very hard time with the downshift trim on my 105 front derailleur (double) on a 2005 Trek 2100 with STI shifters. Trimming up seems just fine - I can move from the small chainring to the large chainring in four distinct steps. But going back doesn't work properly. When I press the small lever to downshift, I have to press pretty hard before anything happens, and then it makes a big click and drops all the way down to the lowest (closest to the frame) position.
The upper and lower limit screws seem properly set, given the overall chain behavior - i.e. if I can get the front derailleur in the right trim position, the chain runs properly without rubbing in all normal positions, and I neither shift the chain off the big or small ring.
The Shimano documentation (at the end, under Troubleshooting) seems to address this symptom, but the "fix" is to "loosen the upper screw 1/8th turn" - that is, to move the upper cage limit away from the bike. This doesn't make sense to me, as I don't see how that's related to my problem, and it indeed doesn't fix anything. It just makes the cage rub against the inside of the chain at the large chainring/small cog position with the cage trimmed all the way out.
On the stand I can maybe carefully get a down-trim click once out of three or four tries, but that's not nearly good enough for real-world usage.
Tips on how to adjust this setting - it seems like some kind of spring tension or something - would be greatly appreciated. Thanks!