Took my 87 Dakota in and it failed 4 times. High HC and high NO. And while the failure points to a bad EGR system, I am absolutely certain the EGR system is functioning as it should. The whole ignition system is completely new. The carburetor is in great shape. The timing is retarded 3 degrees from specs. The truck has passed with this exact same configuration since 2008. The engine is the 3.9L V6 with 170,000 miles, auto trans. Drives like a dream and the AC works!
The only other thing that I can think of besides a bad EGR system is a lean condition that results in lean misfire. The only possible solution that I can think of is to put in larger main jets. The mechanical fuel pump appears to be working. The carb bowl fills when the engine is cranked and the floats shuts off the fuel. Float is adjusted properly. Other than changing the main jets I am flat out of ideas so I thought I would put my line in Car Talk waters to see if I might catch some insight.
I might add that the cat tested good by using an infrared thermometer to read the exhaust pipe going in and coming out, over 100 degrees F. hotter coming out. It is a California approved replacement installed about 4 years ago.
I might also add that manifold vacuum reads 19 to 20 at idle.