I've just bought a shimano xt m770 9 speed mech to replace my half dead LX m570 mech.
No problems at all with indexing the old LX mech and the M760 shifter.
I put the newer mech on and I cannot get it to index. Either it shifts up the cassette fine and then struggles to shift down, or it shifts down, but then has problems shifting up. I had the same problem a few years ago with the same combo.
About half a turn of cable tension each way is the difference between it working one way on the cassette, but not the other.
No problems with the mech (straight, J wheels fine, springs good etc), jockey wheels are lined up perfectly and b tension is bang on.
Ideas?
Have you renewed/replaced the cable - inner at least and possibly outer/s?
Nope. How would this make a difference when it works with the old mech?
Check the mech hanger is straight?
Yep, mech hanger is straight.
In theory Kudos, retaining the existing cable and outers wont make any difference, it's just IME with Shimano, if I'm replacing one or other due to fatigue, Ive always had to resort to replacing the cable.
If everything else is checking out fine, as you suggest, then its a process of elimination.
When you say B-tension is bang on, what are the distances for top and bottom cogs to jockey wheel? Have you tried slackening off the B-tension yet? That may help.
Thinking about it, I replaced the inner about a month ago, when rebuilding the bike.
I've tried the B tension in different positions, but it doesn't seem to make any difference. I'll try slackening it off again and see what happens.
A little less B tension and half a turn more cable tension and it is working a bit better. Still not brilliant, I think it is new shifter time!
Tenacity and perseverance are two very good attributes 🙂
Glad to hear its improving and you have a plan 🙂