Paul, could it be a coil pack ; as each spark plug has its own coil pack on the CSR maybe one of them has packed in. I know this doesn't align with your observation that all plugs were sooty, but if the car had been always running rich that may have been a red herring. Coil packs are sometimes un reliable.
PS a failed coil pack will make it run and sound rough as it is on 3 cylinders, also a cylinder of unburnt fuel will be deposited out the exhaust each stroke so will really smell.
Rob
Edited by - RD on 6 Jul 2012 19:45:55