High NOx is due usually to a defective EGR system , and on vehicles not equipped with EGR systems, the CAT. Often though we find high NOx conditions created due to lean fuel conditions. In this case apparently a smog repair shops has already indicated this. There is no real need to replace the carburetor unless it is heavily worn. If salvageable, a carburetor rebuild kit should seal up internal vacuum leaks. A vacuum leak with in your Chevy C10's carburetor will not allow for the adequate suction of fuel through the metering rods. The rebuild kit should include all required seals, gaskets, and o-rings.
We assume once rebuilding your Chevy's carburetor and adjusting the fuel mixture properly your truck will begin running closer to 14.7:1 air/fuel ratio and lessen NOx production.
