I agree with Robert and Olle.

I've seen it dozens of times; the EGR flows too much for whatever reason, and drops manifold vacuum far enough to cause power enrichment.

CO and NOx are typically caught in a see-saw battle on those old vehicles. You just need to find a way to find a happy medium, and I would start with a properly calibrated EGR valve.

Sincerely,

Michael Barry
Technician
Quick Stop Smog & More
Sacramento, California, USA