nox is usually created by heat so cooling your intake charge can help. You can do this by richening the mixture, but this may also bring up your HC and/or CO. You didn't fail by much, but without EGR, the nox will be a tad high. Water injection helps (if you're into rigging this up), as does running a bit of methanol or denatured alcohol through. Combustion chamber temps are probably too high.