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I have just fitted an 11 - 42 10 spd cassette to my fat bike, replacing an 11 - 36. It's 2x10 with 36/24 chainwheel The smaller one works fine on all gears but the larger one doesn't like the lowest two. The b screw is right in as far as it will go.
Do I need a longer b screw or a couple of links in the chain?
You've just added 3 teeth to the required chain length, so if it was on the short side before, it's quite likely to be too short now. Without wishing to be patronising, if you can't tell whether the chain is too short, it might be worth taking to your LBS as you can do quite a lot of damage with too short a chain.
If it's a FS, then even if the chain length isn't the cause of the shifting problem, you need to check that it's not too short with the suspension fully compressed.