My 94 is running rich it seems. When revved or accelerating it lets off black smoke from the exhaust. I have a K&N filter in as the only mod. I read a thread somewhere that discussed checking the O2 sensor as a possible fix. I can't find that thread now. It mentioned checking voltages off the pick-up if I recall.

Any ideas?

Cheers

Try to check the conections to the throttle control module or to the mass airflow sensor. Check if they are well snapped, or if they have cracks.
Sometimes, the throttle is covered with a black-gray dust, and it causes to give you that black smoke.
Hope this help.