The Environmental Protection Agency is currently undertaking programs that measure the exhaust emissions of in-use vehicles. One of these programs, the Emission Factors Program (EFP), has generated data indicating that a high percentage of in-use 1975 automobiles have exhaust emissions exceeding the Federal emission standards for 1975-1976 light-duty vehicles. Typical failing vehicles have very high CO emissions. High CO emissions may be indicative of improper adjustment of either the idle mixture or the choke. Since idle mixture and choke adjustments are easily accessible and adjusted on most cars, it seems probable that the maladjustment of these two items may be responsible for some of the high emission levels measured in the EFP. In order to further investigate these possibilities, a test program was conducted by the EPA to quantify the effects of various engine maladjustments on exhaust emissions. This test program would help identify maladjustments resulting in the types of failures encountered in the EFP.