Carburetors: Biggest, Baddest & Best Part 3

Dom 1

We’ve mentioned it previously in this series. It’s Holley’s new integrated idle bypass valve arrangement incorporated in the Gen 3 Dominator. Really, this valve is quite similar to what you can find in many mechanical fuel injection systems. In those mechanical injection arrangements, the idle bypass maintains airflow up while the throttle is closed. Not that long ago, there were too hot rod fixes used to increase airflow during idle: The first was achieved by drilling holes in the throttle plates. The other was simply a curb idle adjustment procedure. Here, you’d open the secondary throttle blades by way of the factory set screw. Unfortunately, if the motor has a big cam (long duration and/or a lot of overlap), then you usually have to open the throttle quite a bit in order to gain some idle control. If you open the secondary too much, you expose the transfer slot. With setup, none of those Band-Aid fixes are necessary. That’s not the case here. Holley pre-sets the idle bypass circuit, and it’s extremely easy to tune with a screwdriver to compensate for a big lumpy camshaft. The system is so good, Holley tells us not to set the curb idle screws (first). Instead, work with the idle bypass.

Dom 9
Holley’s Gen 3 Dominator makes use a special idle bypass valve. It’s accessed by removing the carb stud and then set by way of an included screwdriver. See the text for more details.

 

So far so good, but when you check out the photos, something must be missing? There are no extra “knobs” or screws for the bypass. How on earth do you adjust it? Actually, it’s pretty neat. Holley installed the idle bypass valve adjustment screw directly below the air cleaner stud. If the stud is in use for an air cleaner, simply remove it. If you peer inside the stud boss and you’ll note a screw (with a flat head screwdriver slot). In order to set the idle bypass valve, you simply turn the screw. Equally cool is that fact a little screwdriver is included with the carburetor.

Dom 10
In this photo you can see the adjustment capability of the metering block (emulsion restrictors, idle feed restrictors, power valve channel restrictors). The text offers details.

 

The metering blocks fitted to the new Dominator are billet aluminum and they’re designed so that you can tune for just about any condition. As expected, the jets and power valves can be replaced, but the metering blocks also allow for tuning the idle fed restrictions, emulsion bleed restrictors and the power valve channels. Holley notes they determine the idle circuit calibration by the diameter of the idle feed restriction (IFR) coupled with the idle air bleed. The pair of idle feed restrictors is nothing more than a jet for the idle system while the air bleed serves as the airflow-regulating orifice. Holley also states that by turning the idle mixture screw(s), you vary the volume of air/fuel emulsion that is discharged into the intake manifold, not the actual air/fuel ratio. Because of the new metering block configuration, it is possible to change the idle feed restrictor size to compensate for a large (big duration, high overlap) camshaft. With an engine setup such as this, the intake charge is diluted at idle. The dilution is created due to the intake charge being pulled out the header by way of a late closing exhaust valve. In order to return idle quality (along with idle mixture screw sensitivity), the idle feed restriction size has to be increased.

Dom 11
On the secondary side, you can see the metering block includes a set of jet extensions. Note too that the big Dominator does not use a power valve on the secondary side – it’s equipped with a screw-in block off.

 

Power valve channel restrictions are included in each metering block. There is a pair of restrictions, visible when the power valve is removed, however Dominators come with a plug for the secondary power valve. None-the-less, the purpose of these restrictions is to meter the flow of fuel into the main carburetor well. The diameter of the restrictions dictates the amount of fuel that goes into the circuit. When the size of the restriction is changed, the air/fuel ratio will change at wide-open throttle (full power). When the power valve is closed at idle or at part throttle, the power valve restrictions will no effect (obviously).

As noted in a past issue, many of the Gen 3 Dominators are built with an Intermediate Circuit. This circuit provides a smooth transition from the idle circuit to the main circuit. Given the size (copious air volume) of the Dominator, the Intermediate Circuit is engineered to overcome a dip in the fuel delivery curve. The metering blocks used on 3-circuit Dominators include a pair of replaceable Intermediate Circuit fuel feed restrictions. Increasing/decreasing restrictor size delivers more/less fuel to enrich/lean the mixture for the transition (between idle and main circuits).

Holley provides a total of ten emulsion bleeds on each metering block. Emulsion bleeds can have an effect upon power, but really, this is very reliant on the engine combination. Tuning emulsion bleeds must be accomplished on a well-instrumented dyno where it is possible to monitor the air/fuel ratio. By the way, Holley offers a series of emulsion bleed tuning pieces. Contact Holley for more info.

Next issue, we’ll wrap up our look at Holley’s big boy carburetors. There are plenty of features to cover. 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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