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Trying to help a mate out. He’s just bought a 2nd hand bike with a sram x9 shifter and an x7 mech on 10 speed cassette. New chain, cassette, cable inner and outer shifts ok but won’t stay in the lowest ( biggest ring ) at the back, shifts up ok but won’t stay there. Mech tensioned correctly as per various videos. What am I missing? I only use shimano. Is it new mech / shifter time? Also I have found x7 9speed and 10speed mechs but no idea how to tell the difference. I can’t see any serial numbers on the mech. Are they cross compatible?
Thanks In advance.
Have you adjusted the low limit screw?
I've had this occasionally when the inner cable wasn't seated fully during initial set up, and it created a near perfect single shifts worth of slack in the cable, gave exactly the symptoms you describe.