My old truck 1992 ford F 150 with a 4.9 - 300 6 cyl. fowels the spark plugs?

1992 F 150 w/4.9 , 300 6Cly engine. started running rough and fowled up the new plugs thought it may of been the fuel pressure regulator but not sure. I took a fuel pressure test and it shows to be within specs, 69 psi.and holds the pressure ok. The fuel pump never stops working. I thought it had to stop after a few seconds.Also took pressure regulator off and checked out had good vacuum. Replaced spark plugs and started up fine no miss no rough idle. Road tested and got back started running rough and missing again. Black smoke coming from tail pipe. I checked the distributor for wear and checked the pick up coil or hall effect and is working fine. Also checked coil for resistance and changed out the ign. module. The spark seems kinda yellow but jumps to ground good. It starts OK when I clean the spark plugs. No miss fire and Idles smooth. I also adjusted it to the initial timing. Have not replaced the O-2 yet. I know a defective 02 will keep the eng. running in the full rich mode but it shouldn't fowl the plugs. Has anyone had this problem with the same vehicle? could it be the ECM or Injectors? I'm tired of f------g with it. Read the book 10 times but not enough info. Appreciate any kinda help. I'm an old pro. on old autos but don't know enough of the new stuff. Thanks