I've had my Campagnolo Athena 11 groupset now for about 4 1/2 years since I first installed it on my Cinelli, and I've put on quite a lot miles on it, It's generally served me very well. However since putting it on my Genesis Croix de Fer I've had some strange intermittent problems with the rear gear shifting. Over the past 8 months since I built it, I've slowly figured out what was causing some of these problems, it came down to two things:
Derailleur hanger spacing
It may seem to be a minor oversight, but initially assumed that the cassette and hanger would generally be in alignment tolerance. However I noticed it was difficult to shift into the smallest sprocket, it would just sit there indefinitely on the second smallest.
However looking through the Campagnolo manual, it specifies a 10.2-12.2mm clearance between the face of the top sprocket and the face of the derailleur hanger in the diagram below. The Campagnolo cassette on my rear hub was a bit out of spec. Putting a washer underneath the derailleur hanger bolt brought it into spec and this was solved. Lesson learned, there is only a 2mm window of tolerance which really isn't much as it only takes a very small amount of misalignment of the cassette to put this out. The reason for this tolerance is that the spring in the rear derailleur isn't strong enough to pull it into the smallest sprocket if it is outside of this range.
Cable ends and odds
However the second problem wasn't so straightforward. I was having intermittent shifting, sometimes when shifting to a higher gear nothing would happen immediately, or it would skip a gear. These were all symptoms of some cable sticking in the system, however I thought I eliminated that because the cable inners and outers were all brand new. I thought that the shifters had somehow packed it in, despite only having covered 8000km or so. After a bit of googling, I found that the early 11 speed gear shifters were a bit finicky, also in the later models Campagnolo uprated the derailleur return spring as well, suggesting the original spring was a bit too weak. However I upgraded my rear derailleur to a current model medium cage version recently, this noticeably had a stronger spring but the shifting was still slightly erratic.
Recently however, I discovered this post on a forum which blamed poor shifting on the gear cable end rubbing on the inside of the shifter body. A light bulb went off in my head and I remembered that to get the cable ends to fit inside my shifter I had to file them down to get them to fit. The factory Campagnolo gear cable ends are a slightly smaller diameter than the Shimano/SRAM compatible ones I had. I figured that possibly they might be catching the inside of the shifter body causing not enough cable to be released, as the erratic shifting always happened on shifts to smaller sprockets.
I hit it with a file and reduced the overall diameter a little bit, as well as rounding out the end so it had a bit of a dome like you can see in the picture above. This did the trick, and now it shifts beautifully. Every single gear shifts absolutely precisely, I figure now that the slightly larger gear cable end must have been rubbing slightly on the inside of the body, and this would cause the intermittent shifting.
The picture below shows where the cable end would have been rubbing against the inside of the body.
While I was there I also gave the ratcheting mechanism a few drops of oil which also noticeably made both the shift levers feel a lot lighter and more precise. All in all I was stoked that I was able to solve my shifting issues very easily. Again, as with a lot of things, it's the small little details that count...