Engine running very rough
Got my 2001 Focus w/SPI back on the road the other day after replacing the head. The engine ran flawlessly for a couple of days until today..I was driving along and the engine died. I coasted to a stop and it restarted fine but began to run very rough and stumbling after a mile or so. Was close to home and when I pulled into the drive I turned it off and could smell a strong sulphur smell(dont think the cat is stopped up because it looked fine when I bolted it up to manifold). Anyways, I checked for loose hoses/connectors and unplugged most connectors one at a time and restarted only to have it run Very rough each time. Also pulled plugs and they are all black and sooty indicating a rich condition.(plugs are new and looked fine 2 days before) Any ideas what this might be and where to start troubleshooting? Any help would be much appreciated.