Carbon monoxide (CO) is a product of incompleat combustion, which is fully combusted in the catalytic converter by a catalyst of rare earths and high heat (just like HC - same situation, different stuff.) But, if you just replaced the catalytic converter (you didn't tell me, which is why I brought it out first - it's under there and gets knackered about pretty good...) we'll go ahead and cross it off.
The other component that tends to drive HC/CO emissions one way or the other is the EGR valve - but when it fails it drives both of those DOWN. So, we'll leave that aside for the moment as well (yes, everyone, I read the report. It's an 88, so it has an EGR valve. Just wanted to get that out before anyone said it...)
Since the cat and EGR are no longer candidates, the other components which effect fuel metering come into play - the Intake Air Temperature (IAT) sensor, the Manifold Absolute Pressure (MAP) sensor, and the Heated Exhaust Gas Oxygen (HEGO) sensor. Of the three, the HEGO is most prone to failure (heat and crud) followed by the IAT (just crud.) Sometimes a good cleaning will do the job.
If you have an FSM handy, you should be able to find resistance values for various pressures/temperatures, and you can check with a multimeter, using a hand vacuum pump for the MAP and a pot of hot water and a thermometer for the IAT. The only real test of the O2 sensor is of the heating element within, if that's shot you replace the sensor entire. The good thing is that (as I recall) all of those parts are available in the aftermarket, and are significantly cheaper than the EGR valve.
Which one is it? Break out your tools & toys to find out, 'cos I can't tell you from here. I'm going to go out on a limb and say the EGR vacuum circuit is fine, tho, from what I'm seeing of your results.
5-90