excessive carbon monoxide readings.

I have an 87 Jeep cherokee with a 4.0 inline 6 engine. it has 260,000 miles. It actually runs ok except it does not get very good gas mileage. I get about 210 miles to tank of gas. but it recenty failed DEQ. the standard for CO is 1 and my reading was 6.0472. The CO2 reading was 10.5. It passed the HC. I have recently replaced the spark plugs the dis cap and wires. What would be some things that could get the CO reading down. Thanks Don