I'm having a problem with my left twist shifter, model Shimano Revo SL-RS45, indexed for 3 chainrings. It was working perfectly yesterday, but today I noticed it seemed badly out of adjustment - chain rubbing the front derailer in most combinations, not shifting smoothly between chainrings, etc. Nobody touched the bike, the front derailer clamp screw remained tight, and the front derailer cable pinch bolt was still tight.
At the time when I started experiencing problems... There's a little plastic window-cover where it shows a number of which gear you're in, and I noticed that I can't get it to show the number 1 anymore. Gears 3 and 2 show in the window properly, but 1 does not - the number wheel does not shift rotate far enough.
(I have the window cover off in the following photos, so the numbers may not be perfectly centered, but it's close.)
(or what's supposed to be gear 1 - the shifter comes to rest at the 1.5 position)
When the shifter is in that 1st gear (relaxed wire) position, and at rest, I can force the twist shifter another 1/16th of a rotation in an attempt to show the 1, but the number wheel part (and by extension, the cradle that holds the derailer wire) doesn't rotate with it. It's like something is preventing that part from moving back to its normal rest position.
As far as adjustment, I get parallel symptoms. If the cable tension is set such that the front derailer will move onto the middle (2) and large (3) chainrings properly, I can't get it to drop back onto the small (1) chainring when the shifter is twisted back to its resting "#1.5" position. If I add a bunch of slack, so that the derailer is at rest on the small chainring when the shifter is at the "1.5" position, it's impossible to get the derailer to index properly onto the middle and large chainrings. I hope that made sense.
I'm not 100% sure how Revos are internally put together, but I know trigger shifters etc. can get sticky pawls and not want to index properly, so I tried shooting some WD40 into the innards of the shifter. It didn't seem to affect anything, though.
So how do I get my darn Revo to go back to the full, resting, 1st-gear position and show the 1 in the window?