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       The jet number is stamped into the side of each jet. This jet is a number 71, which indicates that the metering orifice is .076 inch.

Idle Circuit

      The idle circuit supplies the air/fuel mixture for engine operation at idle and low engine speed. The purpose of a secondary idle circuit is to maintain a constant fuel level in the secondary fuel bowl. Both primary barrels use identical idle circuits and both secondary barrels use identical idle circuits. However, the primary and secondary idle circuits do differ from each other.

       Primary Idle

      Fuel flows from the primary fuel bowl through the main jets, into a small, horizontal idle-feed passage that leads to a vertical idle-well passage. It then flows past an idle feed restriction, through another horizontal passage, and is eventually mixed with incoming air from the idle air bleed. The fuel then flows down a vertical passage to the bottom of the main body, where it splits in two directions; one path goes to the idle discharge passage and the other goes to the idle transfer passage and constant-feed port. The mixture that flows to the idle-discharge passage flows past the tip of the idle mixture adjustment needle screw, then through the main body and to the throttle body, where the mixture is discharged into the throttle bore, below the closed throttle plate.

      The mixture flows unrestricted in the passages leading to the idle-transfer passage and constant-feed port, through which it exits. When the throttle plate is closed, no fuel is discharged through the idle transfer slot. The transfer slot acts as an air bleed directly above the idle constant-feed port and serves to further lean out the air/fuel mixture.

      As the throttle plate opens and engine speed increases, the idle-transfer slot is exposed to intake manifold vacuum and fuel is discharged from the transfer slot. As the throttle plate continues to open, engine speed and airflow through the carburetor increases; it is increased even further by the venturi effect. As airflow increases, the main metering system begins to operate and the idle system begins to taper off; this provides a smooth transition from idle to engine operating speeds.

       The vertical idle air bleed in the metering block intersects the short horizontal passage that locates the idle-mixture screw.

       Secondary Idle

      All Holley 4-barrel carburetors have an idle circuit built into both the secondary and the primary sides. Some secondary idle circuits are predetermined by design and are not adjustable, but some 4150 carbs with mechanical secondaries have the same idle adjustments that are found in the primary side. Some (but not all) carb models in the 4150 series have idle-mixture screws in the secondary, as well as in the primary, metering blocks. This feature is provided on an individual basis according to the part number.

      An idle circuit in the secondary side controls fuel flow through the secondary bowl; it helps maintain proper fuel level, even if the secondary throttle plates are not open. To set up a carb with both primary and secondary idle-mixture screws, gently screw them in until they seat. Then, back them out one full turn. When the engine is running, adjust the idle speed. Depending on the design of the intake manifold, it is easy to tune for a variety of specific applications with this four-corner mixture adjustment setup.

Main Metering

      When the throttle is partially open, the main metering system on the primary side meters the fuel flow from the fuel bowl through the main jets and into the main fuel well. Fuel flows past the main-well air bleeds, where it mixes with air. This air/fuel mixture then exits through a horizontal passage to the discharge nozzle in the booster venturi, where it mixes with incoming air. Because this air/fuel mixture is lighter than liquid fuel, it responds faster to changes in venturi vacuum and vaporizes more easily when discharged into the airstream. Throttle plate position regulates the amount of the air/fuel mixture entering the intake manifold, and as a result, it regulates engine speed.

       This comparison of a 4160 carb (left) and a 4150 carb (right) clearly shows the secondary metering plate on the 4160 and the secondary metering block on the 4150.

      On the secondary side, the main metering system operates in a similar manner. On 4160 carbs, fixed (or predetermined) restrictions are machined into the metering plate. Idle fuel wells branch off each main well. Fuel for the idle and idle transfer system enters the main well through the plate restrictions and then it travels through the idle well and the idle restriction. Finally, it is mixed with air entering from the secondary air bleeds.

       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 СКАЧАТЬ