[Tom Vaught]

Circuits of a Holley Carb
Holley Carbs (as well as other brands of carbs) have had over 100 years of development time to figure out what is the proper air and fuel mixture during any vehicle/engine load event and what the Air/Fuel Mixture needs to be for most applications

Some writers say there are 5 major circuits in a Holley carburetor, but I would disagree on that simple statement. There are more.
These 'BIG 5 Circuits' are reported as:
The Idle Circuit
The Transition (also called Transfer) Circuit
The Main Circuit
The Power Enrichment (also called Power Valve) Circuit
The Accelerator Pump Circuit
(Everyone seems to forget about the Fuel Bowl(s) and the proper fuel height necessary in the bowl in relation to the different circuits.)
(There are other circuits like the Choke Circuit, the Ported Vacuum Circuit, the Manifold Vacuum circuit, and the Secondary Vacuum Control Diaphragm Circuit that come into play at different times too.)

I have seen Carb articles where writers try to make things simplier for people and segment the different modes of the engine and Fuel System Demand to precise areas. Unfortunately it does not work that way.

You will have "cross-overs" where several circuits are working at or very close to the same time depending on the vehicle event.
But in simple terms this is how it goes:

At Idle, you have the on the Air and Fuel side of the equation you have the Air Passing by the Throttle Blades and the Idle Discharge Port and the lower edge of the Transition Slot Port.

Just Above Idle you have the Idle Circuit functioning and the Transition Circuit functioning, as well as a momentary function of the Primary Accelerator Pump Circuit.

During the Cruise portion of the Drive Cycle you have the Idle Circuit, the Transition Circuit, and the Main Circuit all working together.

During the Power portion of the Drive Cycle you have the Idle Circuit, the Transition Circuit, the Main Circuit, and the Power Valve Circuit involved.

And finally (using the simple discription of how a carb works), during Severe Acceleration Events, you have Idle Circuit, the Transition Circuit, the Main Circuit, and the Power Valve Circuit involved. The circuits act in a "Step Function" as the engine/vehicle transitions thru the acceleration curve to top speed.

How you control (adjust) these circuits is based on where the parts are in relation to each other.

The Basic Air/Fuel Mixture is controlled by the Idle Mixture screws, the Throttle Position Screw, and the Engine Event Profile of the Camshaft. (Radical camshaft events can cause severe Exhaust Gas Dilution of the incoming intake charge at idle).
Mild camshaft profiles will have a much smaller effect on the engine behavior at idle.

The Transfer Slot vs Throttle Blade Position on the Primary side of the carburetor, as mentioned in other posts, needs to be at a precise location for best 'Transition" thru the circuits. You can typically adjust this to a "Rule of Thumb" position and get acceptable performance out of that circuit.

At the same time the engine needs a given amount of air flow thru the engine, typically 15 to 19 cfm, to keep the engine at a acceptable idle rpm. Large engines will get the higher cfm idle flow.

When the engine/vehicle finally get to the Main Circuit and the vehicle is motoring down the road, as you can see a lot of other things happened between the vehicle at rest (engine off) and the engine at Cruise RPM.

You calibrate the Main System based on the Main Air Bleeds and the Main Jets. You can actually dial the circuit in to where you CAN get very acceptable Fuel Mileage from a Holley carburetor contrary to popular believe that the Holley Carb is a "Gas Hog".
Ideally this is by using a smaller Main Jet and dialing in the Power Valve Circuit for the addition fuel required. More on this tomorrow.

So here is where I stop today.

Tom V.