Your Ford Bronco's low CO emissions may indicate a lean fuel condition. The presentation of not enough fuel (lean fuel) will produce low CO and High NO emissions. The reason lean fuel causes NOx is because NOx is created when combustion chamber temperatures rise above 1200f. Adequate fuel keeps combustion chamber temperatures down. When the engine lacks fuel, the chamber temperatures rise, and NOx is created.
Fuel distribution to your Ford Bronco's combustion chambers is primarily controlled by the carburetor. If any component within the carburetor is defective air/fuel ratio can be disrupted and result in incomplete combustion.
The carburetor is controlled by your Ford's computer system. Your engine has a component called the Oxygen Sensor which is responsible for letting the computer know how much fuel is in the exhaust system as it exits the engine. This allows the computer to determine how much fuel it should continue to deliver to the combustion chambers in order to meet the proper Air/Fuel ratio. If for any reason the 02 sensor (oxygen sensor) does not send accurate information to the computer, the fuel delivery program may be altered. The computer will then present either too much or not enough fuel to the combustion chambers and cause an emission failure.
The carburetor has within it, an electromagnetic solenoid (Fuel Mixture Solenoid) which adjusts your engine fuel to air mixture. This component may be at fault. However, keep in mind "this component" only works when supplied information from the emissions control system (oxygen sensor). So with this said, we recommend a smog technician perform a feedback test to ensure the oxygen sensor is in proper communication with the fuel control system, and then perform an accurate carburetor adjustment using a 5 gas analyzer.
