It was a clogged EGR valve btw.. now my NOX levels are down but CO levels back up. I haven't taken it back for smog yet. I'm hoping it's just an adjustment as I've already had the carb rebuilt.

Nice job finding that. I forget that EGR is an issue, mine is factory EFI (fuel injected) which has no EGR.

CO is directly related to a/f mixture. If the HC levels are good, the engine is running well....just a little rich and reading high CO. You can try to lean it out a little but make sure you count your turns on the mixture screw just incase you go in the wrong direction. If you lean it out too much your NOx levels will rise.