Rear derailer cable problems

Find the instructions for read derailleur installation at shimano.com and dig into the technical sections. Follow those instructions exactly. Also, drop the $5 and buy a new cable. Sounds like you've frayed the ends and once they are frayed, those little threads will continue to unwind and cause friction in the housing. You get much better performance from a cable in good condition.

I'm assuming STI shifters here. Bar ends aren't that different. To get the cable out, flip up the hoods. Squeeze the lever a little bit and push the cable out until you see the stop. Remove the cable and replace carefully with a new, lightly greased cable. Then those instructions from Shimano will help you get it installed and adjusted.

Thanks for the information, going to run a new cable FOLLOWING THE DIRECTIONS.

Rick

Did you always have problems w/ keeping the chain running on the largest cog? If so, then you will likely need to adjust the L-limit screw by turning it counter-clockwise. Only turn the screw 1/4 turn or so each time when adjusting/testing. Note: once the limit screws are set, there is no need to really ever adjust them again. Assuming the shop (or wherever you got the bike from) did this properly setup, not sure why the limit screws would be the issue.

If you've just started having problems, then it is likely your cable tension. Cable tension is corrected using the barrel adjusters (the knob-looking thing that your cable housing runs in to in the derailleur). Turn the barrel counter-clockwise if you're having trouble shifting up into larger cogs (this tightens the cable). Do not use the limit screws to adjust cable tension.

Personally, I'd start w/ the cable tensioning first and see if that corrects the problem. If you've tightened the cable to the point where it no longer shifts down into smaller cogs very good and you still can't get the chain to seat on the largest cog, then give the L limit screw a look-see.