Bicycle Mechanics - strange transmission noises...

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

Do you have the correct lock ring for the 11T cog? Shimano makes a specific smaller diameter lock ring for 11T cog cassettes and a lock ring intended for 12T or larger first position cogs is too large and will cause the problem you describe.

Otherwise, the high limit screw may need a minor adjustment.

What he said.

And maybe try a different chain lube?

Do you have the correct lock ring for the 11T cog? Shimano makes a specific smaller diameter lock ring for 11T cog cassettes and a lock ring intended for 12T or larger first position cogs is too large and will cause the problem you describe.

Otherwise, the high limit screw may need a minor adjustment.

I've checked, it's a Hyperglide-C lockring, it doesn't seem to hit the chain, and I've put a fairly new sprocket on. It looks intact, there's no bright metal on any of the faces where the chain contacts the sprocket, so I assume it's not worn. It still skips, and it's definitely not the limit screw either, I tried adjusting that. I also measured the chain stretch, it isn't worn either. What else can I try?

Since you ride in mud, and the chain sounds noisy when wet, I suspect that your chain may be loaded with silt, and may not be as supple as it should be. Also the scraping sound might be dirt on the inner surface of the plate scraping on the sides of the sprocket teeth.

You can often spot any still links by shifting to an aligned gear like outer and third sprocket and backpedaling. Watch the chain as it comes off the lower RD pulley for any links that don't open up perfectly straight. You'll get a better test by pushing the RD cage forward a bit to remove tension until the chain sags just a bit. If any links come out bent or don't straighten immediately, your chain needs a good bath, rinse, dry and relube.

Do this off the bike, using mineral spirits or naphtha, doing multiple rinses until it comes out clean. You can save the used solution in a closed jar - I use 2 one for very dirt wash, and one for cleaner rinse - because the dirt will settle out and clean solvent can be poured of the top next time.

Once the chain is clean, let it dry overnight in a warm place, then relube using a wet lube which will keep water, and the silt it carries out of your chain in the future.

BTW- while the chain is off remove your wheel and try to wind the chain once around the problem sprocket. If you have the wrong lockring, or spacer or a similar problem it'll show up..

I checked the chain, there don't appear to be any stiff links, and I wrapped the chainwhip around the sprocket when I was putting the lockring on after changing the sprocket, it didn't hit the lockring. What do I try now? Should I clean the chain anyway?

A thorough clean and lube can't hurt, won't cost you anything, and will either solve the problem, or eliminate the chain as the source and force you to look elsewhere.