Editing Carburetor (section)

===Power valve===
In a [[four-stroke engine]] it is often desirable to provide extra fuel to the engine at high loads (to increase the power output and reduce [[engine knocking]]). A 'power valve', which is a spring-loaded valve in the carburetor that is held shut by engine vacuum, is often used to do so. As the airflow through the carburetor increases the reduced manifold vacuum pulls the power valve open, allowing more fuel into the main metering circuit.

In a [[two-stroke engine]], the carburetor power valve operates in the opposite manner: in most circumstances the valve allows extra fuel into the engine, then at a certain engine [[RPM]] it closes to reduce the fuel entering the engine. This is done in order to extend the engine's maximum RPM, since many two-stroke engines can temporarily achieve higher RPM with a leaner air-fuel ratio.

This is not to be confused with the unrelated [[Two-stroke power valve system|exhaust power valve]] arrangements used on two-stroke engines.

==== Metering rod / step-up rod ====
A metering rod or step-up rod system is sometimes used as an alternative to a power valve in a four-stroke engine in order to supply extra fuel at high loads. One end of the rods is tapered, which sits in the main metering jets and acts as a valve for fuel flow in the jets. At high engine loads, the rods are lifted away from the jets (either mechanically or using manifold vacuum), increasing the volume of fuel flow through the jet. These systems have been used by the [[Quadrajet|Rochester Quadrajet]] and in the 1950s [[Carter Carburetor|Carter]] carburetors.