Repair Info - Emissions Repair Explained
Overview | Common Issues For Older VehiclesCommon Issues For Older Vehicles
Vehicles that were designed prior to the introduction of emissions controls (1967 and older) are simpler in the sense that there is less to go wrong. Unfortunately, misconceptions about normal emission levels from older vehicles sometimes make it harder than it needs to be for them to pass an AirCare inspection.
Are The Standards Appropriate?
AirCare standards are appropriate to the technology of the vehicle. The thought that older vehicles cannot comply with AirCare standards and run smoothly at the same time is incorrect.
If an engine is adjusted so that emissions are below the AirCare standards but causes the engine to run rough, that is a symptom of a defect. What may cause confusion for some is the fact that many defects can be covered up by richening the fuel mixture. For example, a vacuum leak that disrupts the air-fuel mixture going into one or more cylinders may cause the engine to idle rough. Richening the mixture may make the engine run smoother but it will also cause the carbon monoxide (CO) emissions to be excessive. The solution is to repair the vacuum leak so that the engine runs smooth when the fuel mixture is correct.
About Carburetor Calibration
One of the most common sources of emissions problems on older vehicles is the calibration of the carburetor(s). A carburetor is properly calibrated when the fuel metering components such as jets and rods provide the correct amount of fuel for all of the possible operating conditions. Worn out or mis-calibrated carburetors will result in excess CO emissions (too much fuel).
For any pre-emissions control vehicle, if the carburetor is calibrated correctly, CO levels will be between .30% and 2.00% under light load conditions (such as the AirCare inspection). A common mistake when calibrating a carburetor is to select jets and/or metering rods that cause a relatively rich mixture throughout the operating range rather than just during the operating conditions where a rich mixture is required for maximum performance (acceleration and heavy load conditions). An excessively rich mixture throughout the operating range will not only cause excessive CO emissions, but will also result in higher fuel consumption and reduced engine life.
Summary
The basic combination of an internal combustion engine, carburetor(s), and ignition system has some fundamental characteristics that are common to all motor vehicles, whether it be a 1928 Model A, or a 1965 V-8 with triple carburetors. To function normally, the engine needs to be mechanically sound (good compression, no vacuum leaks), the carburetor must deliver the correct amount of fuel, and the ignition system must produce sufficient spark energy in each of the engine's cylinders at the right time. When an engine and its fuel system and ignition system are functioning normally, the emissions will be well below the maximum allowable levels during an AirCare inspection.
