the knock sensor is probably causing a lack of power due to timing being pulled.
it's a far stretch, but with less power, comes richer a/f ratio.
BUT, you changed to headers, so be sure the o2's are working correctly, that could be the cause of the rich condition, a faulty or misreading o2 sensor.
also, an exhaust leak before the pre cat o2's will cause a rich condition. the ltrims will be very positive, the pcm will keep adding fuel, and never satisfy the lean condition, so it'll run rich.

check those things out and let us know,
allen