1985 Chevy Caprice check engine light - poor fuel economy

I am restoring a 1985 caprice s.W. With 235000 miles on original 305 engine. It runs great and doesn't burn oil. After replacing the carburator with a remanufactured rochester Q jet and having the transmission rebuilt, I noticed a slight hesitation when accellerating ( minor concern ), the "service engine soon" light coming on when DEcellerating and idling for a couple minutes. Scan codes indicated a rich mixture and since the EGR and O2 sensor were recently replaced, it was determined the mixture control solinoid in the carb was defective. It was replaced along with plugs, dist. Cap and rotor, fuel filter and timing adjusted but the "service engine soon" light still comes on the same and my gas mileage is down to 10-12 mpg from 15-17 mpg. My mechanic doesn't know what to try next. Any suggestions?