Join Date: Apr 2009
Location: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
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When erratic shifting occurs only on a single sprocket it's usually difficult to resolve via the adjusting barrel, since that affects all sprockets equally. The erratic shifting you're seeing could be the result of unequal spacing of the one sprocket to it's neighbors, of if it's a loose sprocket cassette, one sprocket turned backward. These issues are rare but of unheard of, and I only suggest them since the cassette is new.
Most likely it's a hanging cable as mentioned by reptilezs. there's a definitive test to verify if this is the case which you should do before anything else.
Shift to the one sprocket larger than the problem shift, then do the problem shift, and stop pedaling if the shift delays or hangs to keep it there. Pluck the bare wire away from the downtube hard like a guitar string, then resume pedaling. Plucking this way pulls down the wire against friction, so if it shifts immediately after plucking then it's dirty, sticky, or worn cables. Repeat the test a few times so verify if it works fairly consistently to confirm the diagnosis.
The other cable related issue could be a fraying inner wire up near the levers, The frayed area will hang where it enters the housing, causing the RD to always stick on the same upshift. This often happens just before the wire snaps, and can therefore be an early warning.
An ounce of diagnosis is worth a pound of cure.
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