casjr2171

Even after replacing the chain on my Sp. Allez (2013) I consistently get a balky, delayed shift at one point when shifting from the largest down to the smallest cog: going from 4th smallest down to the 3rd smallest cog. It's like a "dead spot" in an otherwise smooth run from low to high gearing. Does this sound like a place where one of those homemade "beer can aluminum" shims might fix the problem? The cassette (Sram 12-32) has about 2500 miles on it.

Andrew R Stewart 

Agreed that the cassette is well miled (BITD we could expect 2 or 3 times the miles on a freewheel. But with the thinner stock that the cogs are made of their wear is far faster than back then). And if the cog that is the shifting problem is also the one used frequently...

But I would also look at other system aspects. Cable friction is often inconsistent across the range. Cable wear and casing condition both can cause a problem at only one point through the cable's travel. Another often overlooked wear point is the upper pulley's side slop. Too much and the over shift movement needed to shift to the next cog will increase. The index cable movement can allow this on down shifting only (the lever is pulled past the click point then falls back to it) but in upshifting the cable can only move so much from click to click. A loose cassette on the freehub body can cause inconsistent shifting. Really worn freehub body bearings also cause issues. Andy.

cny-bikeman

One bad spot on a cable could cause such a symptom.

dsbrantjr

First check to be sure that there is not a frayed wire strand inside of the shifter catching on something. Finding that and getting it replaced before it breaks off is WAY better than afterwards.

Otherwise, t sounds as if you may have a kinked housing, inner wire or both somewhere in the cable run. I found this exact problem with a friends MTB recently. If the housing is run under the bar tape it may have become dislodged, making an angled exit from the shifter; it would be easy for a kink to form there.

I'd detach the wire from the derailleur, and keeping gentle tension on the wire, shift to the lowest gear and then make sure that the shifter pays out the wire uniformly, click for click, as you upshift.