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Hi guys...now I'm pretty good at bike mechanics, but this gas me stump.
Just change the rear wheel on my kids OnOne fatty , the new wheel has a XD drive...so to save a bit of cash I got the cassette( 10-42), rear mech and gear changer off ebay.
Now it all works perfectly except when in 42....when it's in this it doesn't change down...no matter how many change downs I do...it won't budge.
If I manually pull out the rear mech to replicate the gear change then all works great again...its just that large 42 ring
Anyone else had this?
New cables ?
B tension screw needing adjusted? Top jockey wheel might be so close that it's not allowing the chain to get up and over the top of the cassette tooth?
I’m going for b screw
12 speed has a plastic thing to line it up. Not sure about 11. But there’ll be a YouTube video some where
It's got a new cable...the b screw is all the way in, but it I can get all the gears it's just changing down from the largest where it just stays put.
All the other gear changes are fine
Mech cage or jockey catching slightly on the cassette in that position? Is the cable actually going slack when you change down? How does the mech feel when you extend it that far off the bike, can you detect any stickiness?
Do you have a gap between the jockey wheel and the 42T sprocket? Is it possible the B screw is all the way in but not up against the tab on the dropout? That is, it’s not pushing against anything. If you unwind the b screw does the derailleur move?
https://www.parktool.com/en-int/blog/repair-help/rear-derailleur-adjustment
Is the cable routed correctly through the derailleur? I had something similar ages ago and think I’d done something random with the cable not looping through the right holds on the derailleur
Does it need a drop link to lower it away from the 42 tooth, I've had these change perfectly on the stand but then be a proper bar steward when out riding ?
Or if it is the B screw put it in from the back for extra effect.
Sorry for the late reply (work)...finally adjusted the b screw to within 3mm of the 42 ring...works a treat now...thank you to all you guys who said B screw ..you are the best 
Do you mean 3mm or 3cm? That looks way too much gap to me, but if it works, maybe it doesn't matter.
From memory it's supposed to be 12-15mm from the top of the tooth on the pulley wheel to closest tooth on the cassette.
But, if it works, it works.
Soz I should have mentioned that that picture is before I adjusted the B screw...
Sram say 3mm jockey wheel clearance but mine is more like 10mm....but it works , so happy dayz.
Have a great day people
When you said you'd got them off ebay, I'm not sure any of us understood that they were secondhand...
Hopefully it was mega-cheap as I don't reckon that there's much life in it.
It's my sons bike, he only does about 300 miles a year on it...by the time it's knackered he should be working by then and can buy his own..lol
Bloody 'ell no wonder it wouldn't change at that angle top pulley has hardly got any chain.(); and I thought I pushed chains and derailleurs to their limits. Ha Ha Ha !