I just tuned (or attempted to tune) up an old Peugeot I got off craigslist last month. Full Shimano SIS (A350) and in great shape. Everything is great except the front derailleur which I can not get to shift to the largest cog. I've adjusted the limit screws appropriately but when I shift up it tensions the cable so tight that the shifter can not click into the next index and immediately falls down to the small ring.
I've tried reconnecting the cable at it's slackest setting and still no luck. Any advice?
If that is not the problem, go along the shifter cable and see where the tension is. If you get to the derailleur, and the tension is still there, see how smoothly the derailleur can move when you push it by hand. It's possible the derailleur is misaligned and running into the chain ring.
If you are certain that your limit screw is not the source of your problem (and you can simply release the cable altogether to check this) then your most likely culprit is not the cable being too tight, but rather, too slack. Check your shifters with the cable released to ensure they are working as they should. Then put your cable back on and get it piano string tight. Should be the cure unless something else really funky is going on ...