How the Metering Rod Work
The metering rods work to restrict, or allow more fuel through the main jets. At idle the metering rods are all the way down into the main jets giving full restriction of fuel. Keep in mind that the metering rods do not fill the main jet hole. The rods restrict, not plug.
The metering rods are assisted by a vacuum piston that pulls the metering rods down during idle and with the help of a spring under the piston, pushes the metering rods up during throttle up. The more throttle, the less vacuum pulling on the piston, the more the spring pushes up on the piston.
Watch a video about the metering rod: