I have a 1983 Dodge D150 with a slant 6 I took it in for emissions this year and failed on:

HC ppm 4082 Allowed 390 Fail
CO% 6.32 Allowed 3.12 Fail
NO ppm 806 Allowed 2796 Pass
RPM 2400 Max 2500 max
CO+C02% 12.4 6.0 Min.

I adjusted the timing 1/4 turn ahead and had it retested now:
HC ppm 259 Allowed 390 Pass
CO% 7.52 Allowed 3.12 Fail
NO ppm 601 Allowed 2796 Pass
RPM 1664 Max 2500 max
CO+C02% 14.3 6.0 Min.

What do I need to do to get my CO% to pass? Any suggestions would be greatly appreciated.

 

Your CO is a produced because of insufficient oxygen for the fuel that's being burned. So increasing the amount of air and/or decreasing the amount of fuel will lower CO. Increase air by making sure air filter is clean or possibly installing an aftermarket air cleaner system with a shorter intake route. Decrease fuel by leaning out the carb as much as you can.

Another source of "fuel" that can raise CO is a lot of oil vapor (from leaking valve seals or worn rings) being ingested by the engine through the PCV. If that's the case, maybe plug off the crankcase vent line during the test?

These are just guesses, hope it helps.