The Holley 4150 carburetor is a four-barrel carburetor designed specifically for high-performance engines. The carburetor is known as a "four-corner" carburetor because it features a single air/fuel idle mixture screw for each of the carburetor's four venturi. The amount of air and fuel used by the carburetor when the engine idles is determined not only by the four mixture screws, but also by the engine's vacuum. Adjusting these screws to set the idle mixture can increase power and fuel mileage, but making the adjustment requires patience.
Connect a vacuum gauge to the intake manifold. Intake manifolds have several small ports designed to provide a vacuum source for many of the engine's components. Not all ports are used however. Unused ports are sealed off with a small plastic cap. Pull the cap off of the one of the unused ports, then slide the hose from the vacuum gauge onto the port.
Locate the air/fuel idle mixture screws. The center of the carburetor is called the "body." A rectangular float bowl housing is bolted to each end of the body. Between the body and each float bowl housing is a second rectangular piece called a metering block, which is approximately one inch thick. One each side of both metering blocks is a single small screw. These four screws are used to adjust the idle mixture.
Start the engine and allow the engine to idle.
Adjust the idle mixture screws. The goal when adjusting the idle mixture screws is to turn each screw until the needle on the vacuum gauge reaches its highest point. Each of the four mixture screws must be turned in unison. In other words, if one mixture screw is turned in a clockwise direction one-half revolution, so to must the remaining three screws. Turn one screw in either direction with a flathead screwdriver and observe the vacuum gauge. If the gauge's needle rose, turn the remaining three screws the same amount in the same direction. If the needle fell, turn the screw the opposite direction. When the four screws have been adjusted so that the needle on the vacuum gauge reaches its highest point, turn the engine off.
Remove the vacuum gauge from the engine and cap the vacuum port on the intake manifold.
Turn the engine on and adjust the speed at which the engine idles. Adjusting the carburetor's idle mixture screws will likely change the speed at which the engine idles. To readjust the engine's idle speed, turn the single screw between the throttle linkage and the body of the carburetor on the driver's side of the carburetor with a flathead screwdriver. Turning the screw in a clockwise direction will raise the engine's speed, while turning the screw in a counterclockwise direction will reduce the idle speed.
