7 speed cassette problem

vicidici

1990 Bianchi
I have just replaced an old Shimano Exage 500X 7 speed cassette with a Shimano CS-HG41 7 Speed Cassette. The old 13 tooth sprocket has been replaced by an 11 tooth.
All larger sprockets work fine but on changing to the 11, the chain is noisy and rides up periodically. I am still using the old chain (new one on order) could this be the problem or is it something more technical?
Any ideas?

sungod

assuming the rd is set up ok, it could be the chain is worn

measure it with a 12" ruler

if the distance from pin to pin over the 12" is >= 12 1/8" then the chain is really worn, normally i'd start planning to change a chain once it gets much beyond 12 1/16"

avoid using it as it'll cause rapid wear to a new cassette

svetty

Chain too long for the smaller sprocket - not enough tension in the rear mech??

keef66

Have you used the old cassette lockring?

An 11 tooth sprocket requires a smaller lockring than 12 and bigger.

The cassette should have come with the right one though