So, I am having trouble with my 89 mustang. It was running real strange at cold start. I decided to change the plugs, and found that one cylinder was really really black. After I changed them, I noticed it ran a little better, but still bad at cold start up (black smoke out of exhaust until warm). After a few days, I pulled the spark plug out of the cylinder that was black. It was sooty again. I changed the wires, cap and rotor, cleaned the fouled plug, and still it sucked. While running, I pulled the wire from the plug, and there was no change.,..... I'm running on 7 cylinders... Without changing valve seals, what else would be a cheap try to fix my problem?