Replaced faulty Shimano twist shifter but front derailleur still won't catch 3rd

Question

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.

Thanks!

Answer 1

That bike, and many similar lower-end bikes, comes with a clamp on derailleur, so the steps to troubleshoot your problem are:

1. Pivots do get dirty; give it a thorough cleaning with degreaser and then drop a few drops of chain lube on the pivots. While you are doing this, dial all the barrel adjusters back to their beginning positions.
2. Check the derailleur cage and make sure it is not bent.
3. Make sure the derailleur is mounted to the frame properly. The cage should be exactly parallel to the chain rings, and it should be placed low enough on the frame that the teeth of the largest cog barely clear the bottom of the cage.
4. With the cable and chain loose, ensure that the derailleur cage moves freely by hand. You can take this chance to begin to adjust the limit screws. Start with the low side, making sure it easily comes to rest to properly shift to the small ring when there is no tension on the cable. At this point it should be clear whether or not the derailleur is functioning properly.
5. With the cable housings detached from the guides Attempt to move the derailleur by gently pulling the cable by hand near the shifters. If this works the problem is likely in your shifters, or with friction in the cable housings. If you can shift it by hand, this is a good time to start to dial in the high limit adjustment.
6. If you make it this far, adjust the cable tension and dial in the shifting with the barrel adjusters.

My experience with that particular bike is that the derailleur is frequently mounted high, is easily bent, and notoriously difficult to keep in adjustment.