Its a 82 Fed. Mazda truck 2.0, carb is a NOS stock Nikki 2bbl
Good score on the NOS carb. Japanese carburetors from the emission control era are notorious for dying very stubbornly (i.e., once past a certain point they are bound and determined to stay dead no matter how much time, effort, and money you put into 'em).
milled the head .060 to clean up some PO's water coolant only on an AL head for pitting corrosion but Mazda spec says .030 is limit and they sell a shim to correct even that.
You may wind up buying that shim kit (or two of them, LOL) but I don't think you're at that point yet.
EGR is clean and functional
Good...
Changed since last test were a valve job and milling the head. perhaps my cat is done. NOx failed by about 10% over max (ie. 70 max, I had 77)
Given those numbers and that set of circumstances on that truck, I think it's right to focus primarily on the emission control system at first.
EGR valve is clean and moves...okeh, but I'd still replace it with a new one. Fun: a whole lot of Mopars use the same valve as your B2000 Mazda. Which means you can swap on the transducer-equipped EGR valve, Standard Motor Products № EGV490 (under $20 at RockAuto). Not only will that get you a smarter valve that's more responsive to changing engine load conditions, but Standard's replacement valves use separate orifice plates so you can dial-in your EGR by swapping the plate. Attached to this post is the breakout chart (Standard's blueprint for the washers). Some valves use the flat plates, some use the cup-shaped ones. Any given valve comes with anywhere from one to several washers; if several, there'll be a sheet listing the correct washer by application. The washers aren't sold as separate parts, but with this breakout chart you can make your own.
You don't mention your numbers on CO and HC (please spit 'em out), but I'm guessing once I see them I will probably call the catalytic converter a prime suspect. It looks like that truck does not have feedback carburetion (no oxygen sensor), is that correct? If so, the cat (meow!) lives a harder life than if the truck had an O2 sensor. Most aftermarket cats (meow!) are cheap junk that'll get you past a few years' worth of inspections if everything else is in perfect condition; they contain
much less catalyst than factory converters and are only rated for a 25,000 mile lifespan. Which is why you go to price a factory converter and it's something ridiculous like a thousand bucks, not worth buying, but Summit will sell you one for $49, equally and oppositely not worth buying. I don't replace a cat (meow!) unless everything upstream of it is in good running condition, otherwise it's just money down the drain because a faulty engine/fuel/ignition system will quickly kill even a brand-new cat (meow!). I buy California-spec converters because the type-approval standards for them are much tougher to meet—they do a better cleanup job on the exhaust and they last a lot longer, so even though they're more expensive to buy, they're worth it in the long run.
Looks like your truck originally had
two cats (MeowMeow!), one directly off the exhaust manifold flange and the other downstream a ways. If both aren't still present, tailpipe emissions are going to be higher than they should be; that's not a terrifically clean-burning engine.
In your case I'd put in
this rear cat (meow!) and
this front cat (meow!). With everything else in good running condition and that new-type EGR valve, that should get you plenty of "breathing" room on the emission test, without having to take apart the engine and shim the head.
