Holley Carburetors. Mike Mavrigian

       With a metering plate, the fuel enters the metering holes at the bottom of the plate. At idle, fuel travels up from the metering holes along the angled “dogleg” passages, through the idle feed passages at the top of the doglegs, and then down through the vertical passages on each side of the plate to the idle transfer slot in the main body. When the fuel moves through the main circuit, it travels from the main metering holes upward through the inboard (slightly angled) passages where the fuel is emulsified by air from the main body. From there, it moves through the main body to the discharge nozzles and into the venturis.

       Note the main metering holes at the bottom of this secondary metering plate.

       Six 1/2-inch 8-32 screws secure the secondary metering plate to the main body on a 4160 carb. These screws are a clutch-head drive.

       All fuel-bowl screws are thread size 12-24 and are available in two lengths. The 4150- and 4500-series primary and secondary fuel bowls, as well as the 4160 primary fuel bowl, use 2½-inch screws (top). The secondary fuel bowl on the 4160 carb (with the thinner metering plate) uses 1⅞-inch screws (bottom).

       Fuel-bowl screws seal to the fuel bowls with fiber or nylon washers. Inspect these sealing washers carefully every time you remove or install bowl screws. If you plan to service the carburetor on a regular basis for tuning or general service, it’s a good idea to have new bowl screw washers on hand.

       The underside of the baseplate has crossover channels, as well as small holes, called curb-idle discharge ports, which are in the throttle body ports under the throttle plates. Engine vacuum pulls a small amount of fuel from the discharge ports as part of the idle circuit.

       The clutch-head screw drive looks like an hourglass. Although you can use a flat-blade screwdriver, you should, more correctly, use a clutch-head driver. A 5/32-inch clutch-head driver is available as a bit to attach to a drive handle or ratchet or as a dedicated driver with a full-size grip. The clutch-head driver engages the screw’s drive and provides secure removal or installation without the chance of damaging the screw’s drive. This full-size clutch-head driver is from NAPA (PN M-135).

       The primary bowl has been removed from this 4160 carb. Its primary metering block is similar to that found on a 4150 model. Note the side-hung float used in 4160 models.

       All Holley secondary metering plates are stamped with a one- or two-digit identification number in the center area. Production numbers, which can be ignored, are stamped at the center bottom. The “9” in the center identifies this plate as PN 134-9; its main metering holes are .067 inch and its idle holes are .031 inch. Metering plates are available in a wide range of primary and idle circuit sizes.

       Shown here are the metering plate from a 4160 (left) and the metering block from a 4150 (right).

       The 4160 (left) uses a thin metering plate between the secondary fuel bowl and main body; the 4150 (right) uses a metering block. This side-by-side-comparison clearly shows why a 4160 requires shorter screws for its secondary fuel bowl.

       On the left is a two-stage power valve with small orifices. On the right is a power valve with a large window passage. The type with small holes is primarily designed to improve part-throttle economy on vehicles with heavy loads and isn’t the best choice for a performance application. The gasket style differs as well. The correct gasket must be used depending on style of power valve. If the gasket is incorrectly matched to the power valve, it leaks. The gasket with a concentric inside diameter is used with a power valve that has the larger window fuel opening. The gasket with three small tangs on the inside diameter seats properly onto the power valve with the series of small fuel orifices.

       The power valve remains closed during idle and normal cruising. The power valve opens when intake manifold vacuum drops to a certain level and load increases to supply additional fuel.

       Here the power valve has been installed in the metering block. A 1-inch wrench is required for removal or installation. The flats on the valve are very shallow, so make sure that the wrench is seated fully. A superior choice is a specialty power-valve wrench (see Chapter 4 for details). The opening vacuum of power valves is marked in inches. This valve is marked 6.5, meaning that it opens when manifold vacuum drops to 6.5 inches.

Power Valve

      During hard acceleration, the increase in airflow results in leaning the air/fuel mixture. A power valve is located in the primary metering block to provide the additional fuel to counter this and to enrich the mixture. The power valve has an internal diaphragm and is operated by vacuum supplied through passages in the throttle body baseplate and the main body. At idle or under load, there should be enough vacuum to keep the power valve closed.

      As manifold vacuum drops under high-speed conditions, the spring in the power valve forces the diaphragm to open, allowing extra fuel to flow through the power valve. The fuel then travels through the restrictions in the metering block and into the main well, joining the main fuel flow, which richens the mixture. After engine speed is reduced, the manifold vacuum rises and causes the power-valve diaphragm to overcome its spring pressure and close.

       The jets and power valve have been removed from this metering block. The thread size of the power-valve hole is 1/2 × 28.

       Whenever replacing a power valve, it’s a good idea to install a new gasket. A power valve should be tightened to the metering block at 40 to 50 in-lbs. Avoid overtightening.

Venting

      The fuel bowl is vented internally to the air horn by a vent tube in the carburetor body, which allows the escape of excess fuel vapors. With the primary fuel bowl removed, you can see a small rectangular passage at the top center face of the metering block. This opening aligns with a