Emission from motor vehicle exhausts of oxides of nitrogen, carbon monoxide, hydrocarbons, and aldehydes is reduced by a combination of enrichment, under off-idle operating conditions, of the fuel-air mixture fed to the engine, and an exhaust converter, e.g., a thermal exhaust reactor. The enrichment is brought about by a mechanical modification of the carburetor, and a selective carburetor control arrangement responsive to engine speed is utilized for enleanment of the fuel-air mixture at higher engine speeds.