This invention describes systems and methods for mixing two fluids. A first fluid, usually fuel, can be passed through a primary passage that typically leads to a carburetor or other inlet to a combustion engine. A second fluid, usually air, can be mixed with the first by introducing it to the primary passage through an inlet located upstream in the primary passage. The mixture of fluids can then be further emulsified by passing it over a plurality of obstructions, such as a threaded interior surface of the primary passage, located within the primary passage downstream of the inlet.