Join Date: Jun 2011
Location: Sunnyvale, California
Bikes: Bridgestone RB-1, 600, T700, MB-6 w/ Dirt Drops, MB-Zip, Bianchi Limited, Nashbar Hounder
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It could be the B-screw is getting in the way of you creating the proper installation. Make sure you pivoted the RD as far clockwise as possible when mounting and tightening the bolt to the hanger. Then the B-screw will clear the stop on the hanger and then rest properly on the hanger. If you didn't, it could damage the hanger where the notch is for the B-screw. That said, make sure the chain routing is also correct. It should wrap counter-clockwise around the rear cogs, then down and around the top pulley, INSIDE the cage tab, down around the left side of the lower pulley, again, inside the cage tab.
Provided you didn't mess with the B-screw before, then it should be identical to where the RD was positioned before, unless of course, the RD geometry was altered somehow by the same thing that forced you to get a new RD hanger. Also, make sure that your RD hanger is installed correctly. If not, make sure your limit set screws are set to prevent the RD from shifting the chain past the smallest cog.