I seem to have developed a shifting problem with my Diverge and I’m struggling to fix it at the moment.

The bike is running 1×11 Ultegra at the moment with an 11-28 cassette. When I’m in the third smallest cog on the cassette the bike tries to upshift every so often and it’s bloody annoying. It tries to shift up and it catches on the bigger cog for about half a revolution then drops back down onto the smaller cog.

So far I’ve checked the mech hanger with a Park Tools gauge, it was slightly out but that’s now been sorted. I’ve removed the cassette and given it a clean, it’s fairly new with no obvious signs of wear or damage. The chain is also fairly new and isn’t showing any signs of wear when checked with a wear gauge. I’ve replaced the cable just in case it was sticking a little.
So far despite checking all of this the problem remains the same.

The most obvious thing now seems to be the possibility of a slightly twisted rear mech but I don’t want to be buying a new rear mech and finding out it’s something else.

What should I be looking at next before I buy a rear mech?

Chain on the right way round? (if you’re running a shimano chain)
cable the correct side of the clamp bolt?

I think it’s a KMC chain but I’ll have to check in the morning just in case.
I’ll take a look at the cable clamp bolt while I’m at it.

Slacken the cable adjuster a fraction? It might cure that problem without upsetting the other ten

Limit screw? Remove the cable and move the mech be hand.

Did you replace inner & outer cable or just the inner?
Also, the 11sp Shimano does seem to prefer the coated low friction cables, standard stainless ones can be a bit more hit & miss IME

Did you replace inner & outer cable or just the inner?

Just changed the inner. The outer is fairly new but it might be worth a try.

Bent tooth on the offending sprocket on the cassette?

Jockey wheels?

Bent mech?

I’ve just been having a play about with the bike in the garage and I think I’ve found the problem.

It seems that I’ve fitted a 10 speed KMC quick link to 1- speed chain instead of an 11 speed one. I’ve switched it over to an 11 speed link and when I’ve tried the bike on the rollers it seems to be spot-on.