Originally Posted by
dddd
What WASN'T done in this case was to back out the B-tension screw to match the smaller freewheel.
The enormous chain gap that resulted from the smaller cogs made the derailer's motions very ineffective at moving the chain from one sprocket to the next.
I've experienced this exact problem. Ran a 12-28 (or was it 11-28?) cassette on a mountain bike once. The cassette profile was flatter in the middle than the arc the mountain-derailleur moved on. Despite backing out the b-screw, I could never quite eliminate the large gap and resulting finickiness in the shifting, especially in downshifting. Some downshifts in the middle of the cassette range required some finesse, some extra holding of the thumb, some minor overshifting. As dddd says, it was all down to excessive chain gap.