1998 Jaguar XJR 1998 Jaguar XJR

Hi, Have just had to replace the engine on my Jag XJR. Fitted a secondhand one but there is a problem with the second cylinder - spark plug gets sooted up quickly and then runs poorly on tick over - could this be valve seals or is it likely to be piston rings? All other cylinders are fine and engine has only done 68K.

Thanks