Recently my 105 rear shifter and derailleur has been ďdouble shifting,Ē most typically when trying to go from the 3rd (21 T) to the 4th (19 T) cog, when it will skip the 4th and go to the 5th (17 T) cog. Probably happens on about 60% of the shifts from the 3rd cog. On those times when it goes from 3rd to 4th as desired, it may very occasionally then skip the 5th cog, going from 4th to 6th. At the same time the shifter has been making a double-click at times when using the large lever to move from, say, the 5th to the 4th cog, though the RD only moves the one gear up, as desired.
So it always pullls in cable and moves to larger (more teeth) cogs correctly (but may double-click), but often skips the 4th cog when releasing cable and moving to smaller cogs.
It behaves as though it sometimes gathers up more cable with the large lever than needed to move the RD (that phantom double-click), stores it for a while, and then releases too much cable on the shift from 3rd to 4th, so the RD moves clear to the 5th cog. Iím not saying thatís what is happening, only that that would have the same effect as whatís happening.
Iím thinking the problem started recently about when I put on a new cassette and chain. The RD was set perfectly prior to that and had been flawless. Iíve tried readjusting it a few times since, but canít make the problem disappear. Not sure what else to look at. I was thinking the shifter might be wearing out (has 18K miles on the rear shifter), but the local LBSs say that probably isnít it, though they canít say what is.
Isolating the shifter by itself with no load it runs up and down the range just fine. If I then just run the cable alone from the shifter and apply light tension on it right where the cable exists the shifter, it shifts fine. As soon as I apply much tension at all on the cable, it will start skipping the 3-4 shift, going to 5. If I pull really hard, I can sometimes get it to skip two shifts.
All the usual online haunts are out of stock of just the right-shifter alone, except for universalcycles.com who has a Dura-Ace singleton. Guess I can live with the skipping until something comes up.