So I've fitted a Hope 40t, think I've got a decent chain length and chain line...but going up from smallest to largest its perfect...but going down nothing happens with 1 click, but then it jumps 2 with 2 clicks.
What am I doing wrong?
I'm rather a novice when it comes to indexing, getting it correct for me is mostly down to luck. PB always say to make sure your barrel adjuster is at its loosest then start indexing from a gear in mid range. Someone will be here soon to correct me 🙂
I hate indexing so much I'm just working to a long term goal of singlespeed.
Barrel adjuster in middle
Shifter in highest gear (smallest sprocket)
Adjust limit screw of mech before attaching cable.
Tension and attach cable.
Shift into lowest gear (biggest sprocket).
Adjust limit screw.
Adjust B screw if needed.
Use barrel adjuster to fine tune.
In your case try winding the barell adjuster IN to reduce cable tension as its shifting down that is the issue.
Shimano or SRAM mech? On the whole block or just on and off the biggest sprocket.
Without knowing more, it sounds like (in order of what I'd check first):
Your cable is a little too tight, if you back off the barrel adjuster can you get it going both ways?
If that doesn't work and the alignment on each sprocket is right as you click up the block it could be sticky cables so the mech is being held from dropping down the block. You can check by clicking one click then pulling the mech down the block by hand to see if there is slack it's not managing to take up.
Or it could be your b-tension. You want to set the jockey wheel as close to the big sprocket as possible while still maintaining a quiet, smooth shift on and off the biggest sprocket. I'm running a Wolftooth 42t sprocket and that won't shift off the biggest sprocket if the b-tension has the top jockey wheel too close to the big sprocket. It'll climb up fine but it just hangs onto it and won't drop down when you click down one click.
My experience has been that SRAM mechs seem to tolerate the big adapter sprockets better than Shimano, but either way the shifting will be slightly slower than with a conventional block because the b-tension needs to be adjusted so far out to clear the big sprocket.