Getting close to binning the lot and buying a new groupset… Another 2hrs spent. Rear hanger was bent so realigned. Rear dérailleur works perfect when on the small cog. Move to large cog (scram force compact) and it won’t shift into lowest cog and makes noise. Adjust so it does drop down happily, but the in small cog it will shift off the cassette completely,

What could cause this? Still the alignment? Or maybe the b screw? As I had to replace rear derailed too.

It’s late and this is only a “what I do” suggestion, no idea if right thing.
I do 90% of mech stuff with no chain and cable. DAG2 tool for rear mech alignment and then set the rear derailleur limit screws by simply pushing mech from smallest to largest on cassette. Make sure you have some barrel adjustment. only put chain and cable on for fine tweak to correctly set b screw tension close as possible and barrel adjust to get chain under cog and snappy shifts. New outers and inners solve many an issue.

Yep, all set up without chain as you suggest then chainon the small ring (34t). Indexing perfect. shift to large ring (50t) and it won’t drop into the 12t on cog rest still indexed fine on rest of cassette. Adjust so okay on 50t means it over shifts when in 34t ring.

Stumped never had so much trouble than with sram force. Fitted an entire 105 groupset a couple of months ago in less time and is absolutely perfect.

Have a look at the front derailleur (FD) alignment, height above the chainring, movement, etc, with and without the chain. It might be that the FD cage is limiting fine movement of the chain in the big ring, which is overcome by releasing the RD too much

Your first post took some reading at this time of the morning 🙂 “ring” and “sprocket” for front and rear is much easier than “cog” for both

Hanger probably not straight. Oh, and it’s a road bike, that’s your problem there

how old is the mech? hold the mech and see if there is any play in the pivots.
if the pivots are worn then whatever adjustments you do you’ll never be able to index correctly.

Sorry few beers and late equal rambling post!

I’ve bought new mechs now so will try with those I’ve been determined not as I wanted to go to 11 speed at some point. Hanger was aligned using a dag2 so it should be straight.