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Chain only runs cleanly when I keep the shifter pressed
(sh.itjust.works)
A place to ask bicycle repair questions, and for bike shop monkeys to share advanced non commercial wrenching resources (no YouTube self promotion). This is only for repair related topics.
What kind of shifters are you running?
With friction shifters, the spring in the derailleur might be pulling the shift lever back. You can fix that by tightening the friction adjustment screw on the side of the lever. Backing out the limit screw might also help.
With indexed shifters, you can try increasing the cable tension a bit (not too much, or it will throw off the other gears).