Holley idle mixture screws

Good picture, as the pic shows, the jets only limit (unless severely undersized) fuel flow at wide open throttle. As air flows thru the venturi of the carb (narrowing of the opening), it creates a low pressure which allows the higher atmospheric pressure in the float bowl to squirt fuel into the air stream and down into the manifold. It seems most people want to change jetting for a rich condition at part throttle. Holley's use a power valve to alter part throttle fuel conditions in conjunction with the accelerator pump squirter.