Abstract: A device is disclosed which, when used in conjunction with a conventional carburetor metering device of an internal combustion engine, allows formation of a mixture of fuel vapor and air which burns more efficiently and yields less harmful combustion by-products than conventional carburetor systems. Incoming air to the engine is substantially completely saturated with fuel vapors by passing the air through a controlled volume of liquid fuel heated to a predetermined temperature, thereby allowing the air stream to become substantially saturated with fuel vapors. The air-fuel vapor mixture is further leaned downstream by the addition of fresh air, and the leaner, but more uniformly and controllably saturated air-fuel mixture then is introduced into the engine.