New chain - VERY loud

I treated my bike to a drivetrain clean, and a new chain yesterday, and it's returned the favour with the loudest chain noise you could ever imagine. The bike is fixed gear, running a 42 x 15 gear ratio.

Chainwheel and cog are 1/8", the chain is also 1/8", and it's a KMC B1S. The chain tension is good (I loosened it off slightly to ensure that it's not too tight), and there was no noise previously.

Does anyone know what might have caused it? Is it just that I've got a new chain on old, slightly worn cog? I've used KMC B1S chains many times before, and I've never had this problem.

I may well have to go back to the old chain (which was a KMC K710), suitably cleaned and re-lubed, if I can't get rid of the noise any other way. Otherwise, is this something that will quieten up with use?

Thanks.

Is it just that I've got a new chain on old, slightly worn cog

that would be my guess

Do the teeth of your chainring or cog have a rough lip on them? usually answers the question of new on old...

Might quieten up with some oil too.

This kind of thread comes up about once a month. Always results in a new cog.

Fair enough - new cog it is then. It was only a cheapy one anyway. Until then, I'll go back to the old chain.....

Thanks.

I had a similar problem with a KMC B1 and I changed it to a KMC Z510-HX and it solved the problem, (my rear sprocket was also old so its not absolutely silent but I can now hear the traffic) I only use the B1's on sturmey Archer hubs where theres never any noise issue on this set up.
Ps the noise was always worst after cleaning and relubing