Jim Smart
December 1, 2006

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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.

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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.