My take is the flat spot could be a lean condition and that will make the NOX bad. If you measure the O2 sensor voltage it should show if you are going lean or not. In closed loop you should get a swing in voltage from rich to lean.

Here is a decent blurb on testing one:

http://www.autozone.com/servlet/UiB...900823d/80/04/f6/02/0900823d8004f602.jsp

If during the flat spot the voltage shows lean you know you have a carb problem.

Milling your head retarded your cam a bit. This makes the overlap period of the cam happen later. It should give you a slightly lower cranking pressure more exhaust contamination of the intake charge at low to mid RPM (lower NOX).

However the tamer cam profile and milled head will decrease the amount of exhaust left behind increasing NOX ( and raise cyl pressures and temps).

I would go after the flat spot first, see if it its from a lean mixture. Alcohol in your fuel will make the mixture even leaner. It still may burn coolerà. But its asking a lot if you are already lean.

I would look for any vacuum leaks or exhaust leaks at the manifold.

I donÆt think you have modified your motor to the point that it wont pass if its running right. I kow of others that have put in torquer cams and passed emissions (that does not say that that putting the stock cam back wont help, just that something else is up).

Good luck,

Kevin