Bicycle Mechanics - Problem with new setup

Hello, I have upgraded by myself my road bike from Sora to 105, except for the chain and the sprocket, both DA. All is working well, but the sprocket in the smallest cog rubs against the second smallest or goes against the drops, up to how I adjust the smallest top rear derailleur screw. It's a 50/34 crankset with a 11/28 sprocket. Also when I start pedalling in that smallest cog the two or three first pedal wheels the chain wants to go to the second smallest, but it returns to the smallest, and all the way it's rubbing and noising. I think perhaps the chain is too long or too short, or the rear derailleur is bent, or the B-tension derailleur screw it's not properly adjusted, but when I tight or untight this screw nothing happens, I haven't tighten it at most but until half the way nothing happens. I have thought also if there has to be some spacer between the 2nd smallest and the smallest cog. Is there any solution? If the chain is too short is it easy to add some link? Many many thanks and sorry for my english.

I've never been able to fix anything by fiddling with the B-tension screw. I am not sure I quite understand the first part of the note, but it seems the problem described later is either cable tension or high gear limit screw. If your chain jumps or tries to jump from smallest cog to next cog, it means either you have too much cable tension or your limit screw is dialed too far in. If, when you shift one click, the chain settles on the second cog without any jump to the third, then cable tension is probably OK and the problem is the limit screw. While turning the crank, dial the high gear screw out slowly until the chain settles on the smallest ring; then adjust cable tension as needed to dial in shifting.

You wouldn't have an issue with too short chain in the big-small combo; that would mainfest in the big-big combo. As for spacers, each cog should be equidistant from the cogs on either side of it and that distance is maintained by spacers for most cogs. But for every cassette I have, no spacer is needed between the smallest and next smallest cog because the smallest cog (11t or 12t) isn't flat like the other cogs; its back is machined so that it fits against the next size cog at the appropriate distance.

If the chain is rubbing against the drop-out when in the smallest cog, you need to add a spacer to the end of the axle. Then readjust the derailler cable tension to align the der with cogs once again.

If the chain is rubbing against the drop-out when in the smallest cog, you need to add a spacer to the end of the axle. Then readjust the derailler cable tension to align the der with cogs once again.

Ohh, is that what the first part was describing? Axle spacer it is.

Sounds like a spacer is right, but I'm wondering what's changed about your rear wheel? I guess you put on a new 105 cassette? Does it have a larger spacer on the inside? Is your wheel in crooked?

Do you have the special lockring needed with the 11t cog? It is smaller in diameter than the normal lockring. If you run the normal lockring with the 11 cog it will interfere with the chain.

Al

Thank you all. When I wrote this I hadn't set the spacer needed between the lockring and the smallest cog, but after that the noise keeps. I think Al1943 you can be right, because the lockring I have is the one that came with the Fulcrum wheel I bought some time ago, and if I need another one, that can be the key answer.

The thing is complicated. I've located the Duraace lockring, but it's not long enough for the wheel. The spacer between the wheel and the biggest cog it's not big enough also, and it seems that I need another type of Fulcrum lockring. If I had know the problems I would have with the Fulcrum wheels, I had passed.