I recently bought a Saracen MTB second-hand, and I've discovered a few strange problems with the drivetrain. Firstly, when I use the smallest sprocket on the cassette, the chain skips, even if I'm lifting the back wheel up and spinning the cranks by hand. I've tried using a different, virtually new sprocket off another of my bikes, and the problem doesn't go away. It's an 11-tooth hyperglide sprocket. Could this be due to chain wear, given that it only affects one sprocket? If not, how can I fix it?
The other problem I'm having is that, if I ride in muddy or wet conditions, the chain starts making a scraping noise. It sounds almost like it's hitting the derailleur cage, but I've checked and it isn't. It only happens after the chain gets wet. It's a Shimano drivetrain, Any ideas what's happening, and is it likely to cause any damage?
Thanks in advance