A NOX failure is often caused by a faulty EGR (not opening or clogged passage) but because of the opposite reason Jmark gave. NOX failures mean that the combustion temps are too high. This is caused by lean mixtures. You have provided almost no worthwhile info to help you out (base timing, vacuum advance hook-up, etc.) A lot of the routine suggestions may help. Leave your PCV hooked up but pull it out of the valve cover so it's sucking in fresh air. Get your base timing set correctly, as mentioned retarding the timing kills performance but often helps pass emissions testing. If your vacuum advance is connected to manifold vacuum switch it to ported, this will allow your timing to stay at base timing when idling. After making all adjustments last but not least set your idle on the high side, not high enough to fail for excessive rpms but just below the max allowed. As far as properly tuning your carb, that I will not get into over the internet. Another major
factor you have not mentioned is whether or not your car is fuel injected or carbed. Either way if it is a computer controlled carb with a M/C solenoid or fuel injected just changing the O2 sensor could get you a passing test.