High Carbon Monoxide Level

I bought my '75 Duster, slant six, auto last year with 55,000 miles. A condition of the sale was that it would pass emissions, which it easily did, even though it ran somewhat rough. Since then, I've replaced air cleaner, plugs & wires, dist. cap, voltage regulator, alternator, & ballast resistor. I know some of those items have nothing to do with emissions, but still, this is what happened. Every reading on the test I just had done( at 61,000 miles) was lower than when I got the car, except for carbon monoxide at idle. It went from .31% to 5.28% (pass is 4.58%). The car runs great, nice & smooth. It is a completely original car, with all smog equipment, & it had a new carb at 48,000 miles. I don't have an emissions analyzer, but I have just tuned it according to the factory specs, & that makes it run rougher. I just can't understand how the readings could jump so much, when I've improved things. Does anyone have any advice? Thanks, - Dale