Originally Posted by
HillRider
Go back and set up the new rear derailleur as if it had just been installed on a new bike. Start with the shift cable disconnected and go through the set up procedure completely including resetting the limit screws, b-screw, connecting the cable with the shifter in the smallest cog setting, etc. I expect that will cure the problem.
Not set up quite right. That's my guess too.
Look up the Park tool instructions for installing a rear derailleur. DO THE STEPS IN ORDER. Each adjustment that you make can impact subsequent adjustments.
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