Restrictions or PVCRs determine the amount of fuel that the power circuit can flow into main well.
Holley carburetors for the street -especially those manufactured for 1973 and later engines with EGR systems -use two-stage power valves (see illustration). Staged powervalves open partially at one vacuum level, then open fully when manifold vacuum falls to a lower level. The first stage in these valves opens a metering orifice smaller than the PVCR. Two stage power valves are used on some replacement carburetors to help certain engines pass emission tests. They’re also available as replacement parts for vehicles with a relatively low power-to-weight ratio, such as recreational vehicles and station wagons.