A system is described for reducing unburned hydrocarbons and carbon monoxide emitted from internal combustion spark ignition engines. The system comprises means for supplying a leaner-thanstoichiometric air/fuel mixture to the cylinders of the engine for combustion, means for conserving the heat of the exhaust products formed on combustion of the air/fuel mixture, and means for controlling the exhaust back pressure under low power demand. The means for conserving the exhaust heat includes the combination of an insulated exhaust port, insulated exhaust manifold and an insulated exhaust pipe; the back pressure control means consists of a valve placed in the exhaust pipe, said valve being responsive to vacuum signal and an exhaust back pressure signal. A method of operating an engine utilizing said system is also described.