I've got a Merlin with Campy Record 8sp (53x39 in front and 13x26 rear). The rear goes 13, 14, 15, 17, 19, 21, 23, 26.
I am having a very specific problem with rear derailleur shifting. When in the 53x21 gear, I try to downshift to the 19 cog and the shift does not complete. With the bike on the stand, it looks like the rear derailleur is not making a complete shift - it's moving to a position half-way between 19 and 21 cog.
The weird thing is that this does not happen with the chain in the small ring (39x21 shifting to 39x19). Nor does the derailleur have any problems in either ring upshifting (going from 19 to 21). Also, if I'm in 53x21 and click twice, it shifts fine into 53x17.
Any suggestions as to what's wrong or where I should start troubleshooting?
Thanks.
-Andrew