One finally admits, however, that «looking at (emissions standards requiring) 0.04 grams / mile of unburned fuel in a ported engine is difficult.» (wardsauto.com)
This produces a bolus of unburned fuel in the exhaust stream. (digitaltrends.com)
They do not have catalytic converters and their emissions of unburned hydrocarbons, oxides of nitrogen, particulate matter, and carbon monoxide are quite high. (builditsolar.com)