Carburetors: Biggest, Baddest & Best Part 2

Dom 1

Last issue, we started our look inside Holley’s latest Generation 3 Dominator. These carbs aren’t simply revised, they’re basically all new, with complete aluminum construction, a taller main body and tons of refinements – all of which prove to be incredibly user friendly. One of the uber-cool features is the air-bleed system. We’ve compiled more info below:

Topside, you’ll note that the air bleeds have been moved outward. Why? It was done so that there is a smoother transition of airflow from the top of the carburetor into the venturi. Like past generation Dominators, the air bleeds are adjustable. The Dominator has 12 air bleeds (Holley’s 4150 HP Ultra series has 8 air bleeds). The difference between the 4150 and the 4500 series Dominator is due to the fact a Dominator incorporates a separate Intermediate Circuit (three circuits: Idle, Intermediate and High Speed). In a Dominator application, the four outboard air bleeds (primary and secondary sides of the carburetor) are for the idle circuit. Alongside are the intermediate air bleeds. The four innermost air bleeds (closest to the bowl vents) are for the idle circuit. And yes, it is possible to remove and replace all of the air bleeds, but Holley advises that you leave them alone, unless you have access to a well-instrumented dyno for testing. If the bleeds are changed, Holley recommends they should not change more than six sizes in any one direction from the original size.

Upstairs you can see a series of bleeds – Dominators with intermediate circuits include three on each side of the respective bowl vents for a total of 12, and they’re all adjustable.
Upstairs you can see a series of bleeds – Dominators with intermediate circuits include three on each side of the respective bowl vents for a total of 12, and they’re all adjustable.

 

So far so good, but what’s the real purpose of the air bleeds? According to Holley, high-speed air bleeds are used to emulsify the fuel prior to it entering the discharge nozzle (where, obviously it is discharged into air stream within the venturi). As the air bleed size is increased, the air/fuel mixture leans out. When size of the high-speed air bleed is decreased, then pressure across the main jets will decrease. This in turn, pulls more fuel through the main system, which translates into a richer mixture. The high-speed air bleeds act as anti-siphon devices. They prevent fuel from dribbling into the venturi when the airflow is reduced or stopped (for example, when the throttle is closed). Keep in mind that as engine speed increases, the air/fuel mixture must be a bit rich. This prevents high speed lean out, which (obviously) can have catastrophic consequences.

Look closely at each of the venturis in the accompanying photo. See the little slash cut tube that enters the venturi?  That’s the discharge for the intermediate circuit. The text offers more info.
Look closely at each of the venturis in the accompanying photo. See the little slash cut tube that enters the venturi? That’s the discharge for the intermediate circuit. The text offers more info.

 

New Dominators are available with two or three-circuit metering. Two circuits simply refers to the fact the carburetor has an idle and a high-speed circuit. A carburetor with three-circuit metering adds an intermediate circuit. Here, the extra circuit draws fuel through the metering block and air from the bleed in the main body. The air and fuel then enters a tube that discharges the mixture into the throttle bore. What happens here is the intermediate circuit richens the mixture. For a street driven car, that might work that well, but if the car is a drag race piece that’s launched off a trans brake or if the application is a stick shift combination, then it can prove to be an advantage. Three circuit carburetors are typically drag race only. You’ll find that Holley offers a choice of two or three circuit arrangements in 950, 1050 and 1150 CFM Gen 3 Dominators. Larger 1250, 1350, 1425 and 1475 CFM Gen 3 Dominators are all supplied with three circuits.

In this photo, the pointer shows the idle circuit air bleed.  There are four of them – all are closest to the bowl vents.  If the idle air bleed size is decreased, the mixture richens by increasing the pressure drop in the system.  There’s more info in the text.
In this photo, the pointer shows the idle circuit air bleed. There are four of them – all are closest to the bowl vents. If the idle air bleed size is decreased, the mixture richens by increasing the pressure drop in the system. There’s more info in the text.

 

When it comes to the idle air bleeds, keep in mind the idle system provides fuel at idle and low engine speeds. When the idling or at just off idle, it demands a richer fuel mixture than it does at high speed or part throttle. When the idle mixture is lean, then combustion becomes slow and irregular. That’s what contributes to a rough idle. If the idle air bleed size is decreased, the mixture richens by increasing the pressure drop in the system. If the idle bleed is increased, that leans the idle mixture by reducing the pressure drop across the idle air bleeds. Holley advises you can actually accomplish the same thing by simply backing out the idle mixture screws. This will increase the pressure across the idle air bleeds, which has the effect of pushing more fuel from the idle well. The result is a richer air/fuel ratio. Holley recommends you use the four idle mixture screws to adjust the mixture. If at that point you still have issues, then you work with air bleeds. For a closer look, check out the accompanying photos.

Next issue, we’ll examine the idle by-pass system. It’s a slick concept that works very well. Watch for it.

Holley Performance Products

1801 Russellville Road
P.O. Box 10360
Bowling Green, KY 42101-7360
Website: www.holley.com
PH# 270-782-2900

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