Black smoke, erratic behavior

This refers to an 88 Sentra with a throttle body. I have been experiencing black smoke for several weeks with erratic engine behavior including lack of power above about 1500-2000 rpm that worsens as the engine heats. I completely changed the throttle body, changed plugs and wires, swapped computers, even put in an oxygen sensor when this all began. Nothing seems to help. The engine remains full of carbon and the plugs black. I'm about to give up on it. Any ideas?
Thanks,
Dan

Checked the fuel regulator? Excessive fuel pressure could be causing an extreme rich condition....

is the egr valve stuck open????????

Hello. i too have a nissan sentra but it's a 1987. I also experience sputtering at high rpms and it gets worse as the car warms u p. I travel 75 to 80 miles per hour to work and it really gets bad at 75. especially when i maintain an even speed at that mph. I have heard that the fuel regulator is suspect. For the throttle body they're $109.00 to $124.00 depending if their is one or two lines on it. I'm taking my car to a nissan dealership tomorrow for them to plug in and isolate the problem. I'll keep you posted.

Having taken my 1987 sentra into a dealership to diagnos a similar problem, the computer was kicking out a code for the isc valve (aka IAC motor). It is located on the back of the throttle body. It can be purchased aftermarket at autozone for $130.00. I'm buying mine tonight.

Also check if your catalytic converter is clogged.. its an 80's car never know whats in there LOL. but yeah check the cat