I'm doing a modern conversion on an old steel frame, and it's working out quite nicely. I have, however, run into an issue with 11 speed indexing. A few of the cogs just won't index, but the same components work fine on my other bike. The rear triangle is 126, so I'm spreading the rear triangle to insert the wheel. In theory, this would mis-align the derailer hanger slightly, which must be enough to impact shifting negatively. It's aligned well enough for 126 because it indexes fine with a 126 wheel, and works fine with a 130 wheel and 8 speed. My guess is the tiny difference is throwing off the shifting for 11 speed. By the same thought, my plan to cold set and align the rear end to 130mm would solve the problem, right?

Which cogs won't index? I'd have thought the high cogs, small ones, would be accessible by the dérailleur but perhaps not the low gears.

It wouldn't shift to larger cogs from the smallest position, or smaller ones from the largest few. It wouldn't shift larger at all, and would skip in twos going smaller. Adjusting the barrel to fix the problem just buggered up the rest of the shifting.

What is the condition of your cable housings, particularly the last run going into the derailleur? Is that loop the correct length, neither too long nor short, clean and un-worn? Your symptoms sound like cable friction, although if you suspect derailleur misalignment that should be checked and corrected first. I replace cables and housings a lot and am often surprised at how much improvement (in terms of ease of adjustment) occurs with a system which seemed to be working fairly well before the change, even with 7- and 8-speed systems. With an 11-speed system everything needs to be in tip-top condition for reliable results.

its all new. I put fresh cables on when I set this bike up since the routing is completely different than the other bike and a little longer as well. Strangely I tried again and it seems to work. I'll find out when I ride the bike. Still, I'll have the rear end aligned to 130mm spacing so there's no oddness.

Sometimes the devil is in the details.

For indexing to work properly, the derailleur has to operate in a plane that is exactly parallel to the cassette cogs. 11-speed cassettes pack 11 cogs into roughly the same amount of space that an 8-speed cassette only has 8. Since the cogs are closer together smaller amounts of misalignment will have a negative effect on your indexing. I suspect that the more precisely you can get your bike's rear triangle, dropouts and derailleur hanger aligned, the more reliably your shifting will index.

That was exactly my thought. With 11 speed, it has to be so precise that bikes straight out of the box sometimes need aligning. I already planned to have the rear end spaced and aligned so it's more convenient to swap wheels, so it ought to be perfect. I just got back from a ride with the bike set up with 11 speed, 130 wheel just shoved in. It worked fine, but there are a few shifts that would be marginally better with alignment.

You Had to " Upgrade " it was simpler Before ..

Might be time to Just Buy a New Bike to get all those Bells and whistles .

I've got plenty of those too. Since I found a better bike to have as a vintage bike, this one didn't fit in my collection anymore. It was too retro to get ridden regularly and too flexy in the front to ride hard, but it rides so well. It came to me with bad paint and lacking a lot of the original parts, so in my head it's a perfect candidate for a resto-mod.