 As the throttle opens, we...  As the throttle opens, we cruise from the bottom idle port to the idle port above the throttle plate. The more we open the throttle, we're handing fuel delivery off to the power circuit, where fuel enters the throttle bores (also called venturis) via the boosters, getting the fuel into the mainstream-bore air flow. |  Here's another example of...  Here's another example of the idle circuit in an Autolite 2100 series two-barrel carburetor. Fuel is drawn from the fuel bowl to the idle port below the throttle plates. As we open the throttle, fuel delivery transitions from the bottom idle port to the off-idle port above the throttle plate. Some carburetor throttle bores are slotted where the plate closes. This allows a smooth transition from idle to power circuit. |  The idle-transfer slot on...  The idle-transfer slot on a Holley 4160 handles fuel delivery to the power circuit. Think of it as the off-idle port above the throttle plate. |
Cruising/Off-Idle Circuit
When the throttle is open and we're off the idle and off-idle circuits, fuel delivery changes considerably. With the throttle plates open, fuel is not drawn from the idle and off-idle ports but through the bores and boosters. Air flow through the boosters is what draws fuel from the bowl via the main metering jets. This works because the throttle bores are tapered mid-section, like an hour glass, to increase velocity at the boosters. With the engine running, quickly crack the throttle to wide open, and watch fuel spray from the boosters into the throttle bores. The engine will respond accordingly. The open throttle plates allow intake-manifold vacuum to draw air from the atmosphere and fuel from the bowl via the boosters. This makes the engine rev.
How we meter fuel from the bowl depends on the carburetor manufacturer and model. Holley 4150 and 4160 and Barry Grant Demon Series carburetors have two main-metering jets located in the metering block at the base of the fuel bowl. Jet size (inside diameter) determines how much fuel will flow to the boosters when the throttle is open. Edelbrock and Carter AFB/AVS carburetors have vacuum-operated main-metering jets and rods that control fuel flow when the throttle is open. The same can be said for the Rochester Quadra-Jet four-barrel carburetor found on '70-'71 429ci Fords. As we open the throttle, intake-manifold vacuum moves the tapered rods which control fuel flow. Some carburetors have mechanical fuel-metering rods that move with throttle movement, metering in more fuel.
 When we adjust the idle mixture,...  When we adjust the idle mixture, this is exactly what we are doing. These needle valves adjust the amount of fuel flowing to the idle ports at the throttle plates. When we close them, we cut off fuel flow. When we open them, we increase fuel flow. The idle-mixture adjustment controls the air/fuel ratio we are getting at idle speed. It has no bearing on mixture when the throttles are open. A weak spot of Holleys is the metering-block passages for the idle circuit. When they get clogged with dirt, idle quality suffers. What's more, it doesn't change when we turn the mixture screws in. |  This is a basic example of...  This is a basic example of the cruising or off-idle circuit. With the throttle open, vacuum is created through the throttle bore or venturi and draws fuel through the booster in each bore into the slipstream. The fuel is atomized (vaporized) and flows past the throttle plates into the intake manifold. |  In this cross-section of a...  In this cross-section of a carburetor, note the booster. As its name implies, the booster helps accelerate the flow of air through the bore, giving the fuel particles velocity. It also improves the fuel-spray pattern. |
Power Circuit
Whereas Edelbrocks and Carters rely on a system of metering jets and rods to keep the boosters supplied with fuel in wide-open throttle conditions, Holley and Demon look to the power valve. The power valve is vacuum operated, designed to allow more fuel flow when the vacuum signal reaches a given number of inches. Holley and Demon offer a variety of power valves, numbered for identification based on the vacuum signal. We choose a power valve based on the vacuum level at which we want additional fuel. This can be a tedious process of trying different power valves until achieving the desired performance.