i have a 1977 g20 van. it doesn't require a catylitic converter (as by the emission label) .it also has a air injection pump.i failed the calif. smog test because my NITROUS OXIDE readings were 3333 ppm @ 15 mph & 2611 ppm @ 25 mph. what can cause these readings ?

 

high combustion temps cause nox to form. the egr system is designed to lower the temps by introducing exhaust gases back into the fuel charge. Inspect the egr system to see if the valve is opening and the passages are not blocked.