I have one of the ubiquitous Wal-Mart-sourced Chinese Kent bikes, badged as a "GMC Denali." It came with the Shimano RS41 3x7 twist shifters.
The front shifter got to where it would not physically move all the way into "1," such that if I set up the front derailleur I could catch 1 and 2 but not 3 (I presume the shifter broke inside and a loose piece was jamming it - it wouldn't move all the way to 1 even if the cable were disconnected at the other end). Earlier tonight I replaced both shifters and cables with a brand new RS45 set (I was told by the vendor that this would be a straight replacement) and whereas I was able to set up the rear perfectly, in the front I still couldn't quite catch 3. It appears as though the front derailleur simply will not mechanically move far enough outward to move the chain to the largest sprocket, even with the high limit screw adjusted all the way out. I can play with the barrel adjuster at the shifter all I want to get it to where the shifter can be forced into clicking into 3 with the cable as tight as a guitar string, but the derailleur simply isn't going to go any further (and yes, it is high enough that it is clearing all the front sprockets).
Might this derailleur have worn out its pivot(s) such that its range of movement is reduced, or do I need to adjust the low limit screw until it almost won't go into 1 anymore and try again? I'm perfectly willing to replace the clamp-on derailleur if it's just gone south, but I sure don't want to if I'm just missing an adjustment nuance.