Originally Posted by
silver_ghost
Try flushing out the shifter with some WD-40. No need to disassemble (in fact, don't), often the factory grease inside becomes gummy with dust and keeps the mechanism from engaging. Usually a quick flush will clean things up inside.
+1 I recently got a '91 Giant Yukon MTB in shape for my son to take to university. Both RapidFire shifter would only engage a couple of the indices. I gave them both a good flush with the WD-40. The rear shifter perked up immediately. I had to give the front shifter a couple of days and a repeat, but it perked up nicely.