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I'm assuming that the chain is stuck on the second smallest cog. To check if it is the limit screw that is the problem while turning the crank try and push the pulley cage over to try and make the chain go down. If it does shift than it is the limit screw that is the problem so you can adjust the H limit screw. If pushing the cage doesn't make it shift than you might a cable tension problem which you could fix using the barrel adjuster. Good luck!
How to tune your back derai...View Instructable »