Sram Force not shifting into the largest sprocket

I've just finished (well not quite) setting up shifters on my bike. All new cables and outers, chain and cassette with used Sram Force shifters and rear derailleur. Shifting is super smooth and switching from Shimano I really love double tap.

One problem though: I can't shift into the lowest gear / largest sprocket in the back. When I try, it shifts up and immediately back down to the second largest sprocket. The shifter makes a noise which sounds like the usual upshift sound, directly followed by the downshift sound. It feels almost like after the upshift something forces the downshift. I can upshift and hold the lever in the position and it will stay on the largest sprocket until I let loose. The shifter also behaved that way, even when it wasn't hooked up to any cables.

Rear derailleur h+l screws are setup correctly, cable tension is fine, derailleur hanger is not bent.

That leads me to think either something with the indexing in the shifter is broken. Which I have no idea how to troubleshoot and fix. Or something with the derailleur and cassette combination/setup is off.

In case of the second option, it might be related to this: after shifting down to the smallest sprocket, I can make an additional shift down, which leaves the chain on the smallest sprocket and doesn't push it off the cassette. It almost feels like a half-shift. Is this a trimming feature or setup error? If the issue is related to that, could it be that the half-shift would actually shift into the smallest sprocket if setup correctly? Maybe because of a missing spacer or compatibility issue with the cassette or hub? I'm not using any spacers on the hub.

Components:
Sram Force DoubleTap Shifters (10x Road)
Sram Force Derailleur (10x Road)
Shimano CS-HG500-10 (10x Mountain)
Shimano XT FH-M756A Rear Hub (10x Mountain)

Thanks for your help!

Sounds like you need more cable tendion, Sram likes having the cable pulled as tight as possible before clamping it to the derailleur. Try unclamping the cable, set the barrel adjuster so it's got half to a full turn spare to decrease tension if needed, then pull the cable tight and at the same time push the derailleur a few mm in-board, then clamp the cable. If you've pushed it too far when clamping the cable you can release a bit of tension with the barrel adjuster.

I think the cable might just not be tight enough. You might have set it up when the shifter was in the position of gear 2.

Loosen off the cable at the clamp on the rear mech.
Apply some tension to the cable by pulling at the mech end
Whilst maintaining that tension, shift all the way down as though you were shifting into the hardest gear
Tighten the cable up in the clamp and shift into gear 2 by tightening the barrell adjustor.
Whilst pedaling loosen off the barrell adjustor until it shifts back down to gear 1

That might fix it

edit: seems we have the same solution

after shifting down to the smallest sprocket, I can make an additional shift down

Is your indexing not 1 sprocket off? Shifter in cog 2, when the chain is on cog 1?

Easy test without undoing the cable, wind the barrel adjuster out several turns and see if it has improved at all

You all are right, the indexing was off. So much for overthinking [facepalm]
Tested it by winding out the barrel adjuster, which improved things. Eventually reattached the cable with more tension and voilá.
Thanks everyone for your help!

I’m having sort of a similar problem.

I’ve followed the instructions in the park tools video for adjusting rear derailleur step-by-step.

It all runs smoothly up until the top one, where it doesn’t seem to want to go in and jumps between the largest and second largest cog. If it then does eventually go in, when I try to shift back down, nothing happens until I shift down another click. Then after that pretty much all the cogs have excess noise and I feel like all the adjustment I just did has gone out the window.

I guess something is up with the indexing but I just tried the steps above re releasing the tension, shifting down and tightening back up again and the issue persists.

Any suggestions?

I should add it was all working fine before and I only took the chain off to clean it, and once I put it back on all this happened. Thinking maybe the insane amount of pulling with wire to release SRAM quick links has pulled it out of shape.