Originally Posted by
stausty
These shifters can get gummed up with dirt and soot and such so that the pawls don't engage and the shifting no longer works correctly. You can clean them out by spraying degreaser, WD40, or something similar into the shifter body while moving the lever through the gears. Hopefully if you can clear out the gunk the shifting will work again.
Yep. My old LX shifter acts the same way. Stick the WD40 straw in the cable hole and give it a good flush. Good as new (as least for a few months)