For controlling the amount of fuel pushing through the metering jet some carburetors (mostly Carter), use metering rods. The metering rod is used to control the size of the opening. During low RPM the metering rod is down the large step in the metering rod blocks much of the opening. When up and at higher RPM the rod raises and allows more fuel through.
Metering rods are controlled in different ways.
1. Mechanically - The metering rod is connected through a series of levers and rods to the throttle. Pressing on the gas raises the metering rod.
2. Vacuum Controlled - The metering rod is connected to a vacuum piston or to a diaphragm. The piston is spring loaded to keep the piston up. When vacuum is applied the piston is pulled down closing off the jet.
3. Both Mechanically & vacuum controlled as in the illustration below. The metering rod is hooked to the diaphragm pull rod which is being held down by vacuum from the manifold acting on the diaphragm. The pump lifter also passes under the metering rod arm. Either loss of vacuum or full opening of the throttle valve will cause the metering rod to be lifted.
Looped Metering Rod
Another type of metering rod is a solenoid controlled type, or looped. A current passes through a solenoid, which is used to move the rod. The rod position is controlled by fuel needs.
An oxygen sensor in the exhaust stream and a control circuit energize, or deenergize the rod solenoid as needed.
