This paper on vehicle emissions contains actual data from 19 vehicles, taken with the intent of viewing both emissions and control device deterioration with vehicle age. In addition, valuable insight is given to the value of the 2500 rpm no-load test for emissions.
It was found that the 2500 rpm, no load, is not sufficient load and/or engine speed to cause the throttle to open in all cases. The most obvious effective emission control device observed were those creating a lean air-fuel ratio, retarded spark at idle, and dashpots on the carburetor to stop rapid closing of the throttle on deceleration. These are described in detail.