High concentrations of CO interfere with thought processes complicating the diagnosis. Carbon monoxide is a cumulative poison which can rise to harmful levels in the body in minutes. In 1968 the EPA regulated CO emissions from on-road motor vehicles. Engines used indoors were not originally regulated. Recent regulations affect small engines such as those used on lawn mowers, chain saws, weed eaters, electric generators, water pumps, and boats, although the regulations for small engines will continue to allow considerably higher CO concentrations than the tighter regulations for on-road motor vehicles.
Carbon monoxide is produced during incomplete combustion.
Anything that leads to incomplete combustion increases CO production. A gasoline engine producing 10,000 ppm CO at the ideal air-fuel ratio will produce over 60,000 ppm when the fuel is increased. The distribution of a liquid fuel, such as gasoline delivered through a carburetor, is not uniform. To ensure all cylinders obtain sufficient fuel to produce maximum power, the mixture must be rich. Excessively lean mixtures can cause engine problems, although fuel economy is improved with a lean mixture.