CO stands for Carbon Monoxide. Having failed for high CO is more then likely due to a "rich" fuel mixture problem. The primary cause for a rich fuel mixture is a defective carburetor. If any component within the carburetor is defective air/fuel ratio can be disrupted and result in incomplete combustion; resulting in high CO, and usually high HC as well. You can ask a local smog tech to try and adjust the carburetor or you can ask to have the carburetor rebuilt (a little more expensive but the performance increase will definitely be noticeable).
We recommend finding a carburetor expert to work on your Toyota's carburetor. There are 2 (visible) screws on the carburetor for idle/speed adjustment. The screw that is screwed in furthest is for your Toyota pickup truck's idle speed. The other screw is for the high idle cam speed. If you have idle speed problems, you should start here. We also recommend checking carefully for any vacuum leaks around the carburetor, a stuck carburetor float, and air/fuel mixture proper adjustment. The air/fuel mixture screw is hidden. Toyota does not want owners tampering with it. The idle/air mixture screw is located on the passenger side of the carb and has a small metal cover about the size of a pencil eraser. It must be drilled out in order to gain access to the screw. You may then adjust the air fuel mixture. Turn clockwise to lean the air fuel mixture, and counter clockwise to richen it. Typically factory adjustment is the air/fuel mixture screw turned all the way in and then backed out 2 and 1/2 turns.
Fuel distribution is also controlled by your Toyota pickup's computer system. The 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.
