1987 Chevy Camaro Emssions Test Failure

Engine Performance problem
1987 Chevy Camaro V8 Two Wheel Drive Automatic 136K miles

I resentlybrought my car to a state that requires Emissions test. First time the car ever had one. Everything pased with near perfect results accept for the NOx levels. The limits are 1047 and I failed with reading of 1792. The car has a new catalytic converter, did a tune up and oil change. This car is also carburetor equiped with a quadrajet. Also, the check engine comes on when idle for a long period of time and goes out once driving again. Originally the car was dual exhuast and was made in to a single by the original owner who could not afford to replace the entire exhaust sytem.