Unless you already know all about Holley metering idiosyncrasies, these emulsion circuits should be left to professional tuners. Just so you know, the basic function follows that increasing the size of these emulsion holes will reduce fuel flow and lean out the overall fuel curve. Increasing the size of the high-speed air-bleed will also delay the onset of fuel flow in the main metering circuit. Reducing the diameter of the high-speed air-bleed produces the opposite effect.
Think of the emulsion tube in the main well as a straw immersed in your favorite soda. A light amount of suction (pressure differential) on a straw with no holes pulls a large amount of liquid very quickly. What's really happening is atmospheric pressure pushes the liquid from the glass into your mouth. If you place a small hole in the straw above the liquid level, this introduces air into the straw, requiring a greater pressure differential (or more time) to move an equal amount of liquid up the straw. The liquid that is pulled up the straw will have tiny air bubbles mixed in with the liquid. What you've created is a simple emulsion tube. The difficulty comes in figuring out the size, number, and placement of emulsion holes in the metering block. The good news is that Holley has worked all this out for you, but at least now you know what those holes do and how important they are to an engine's fuel curve.
The classic Holley 4150 carb as a mechanical secondary is represented by this 750-cfm carb
Holley HP Vs. Street HP Vs. Standard
When the specialty carburetor market exploded with tuners modifying basic mechanical-secondary street carbs for competition, Holley responded with the HP series with a new contoured venturi main body equipped with screw-in air-bleeds, stainless steel throttle blades with buttonhead screws, strengthened metering blocks, and Dominator-style fuel bowls. Following the HP, Holley produced the HP Ultra version, which upgraded to black-anodized billet metering blocks with screw-in idle feed and emulsion jets and a billet baseplate. The HP Ultra is an expensive carburetor, so Holley introduced a much more affordable Street HP 750 in the bright finish in either vacuum-secondary (PN 0-82750) or double-pumper (PN 0-82751) configurations. These carbs offer screw-in air-bleeds and a street/strip calibration that is leaner than the HP and Ultra. The vacuum-secondary 750 Street uses a two-corner idle circuit, while the 750-cfm double-pumper offers a four-corner idle. The original HP carbs are available in sizes ranging from 390 to 1,000 cfm in the 4150 configuration. The HP Ultras come in only 650-, 750-, and 950-cfm sizes. There are also Dominator HP and HP Ultra versions.
Carburetors are required to create a stable air/fuel ratio over an incredibly wide range of rpm and load. HP carburetors come with adjustable air-bleeds and idle feed restrictors that make it much easier to fine-tune the fuel curve to your particular application. By using screw-in idle air-bleeds in the top of the carburetor, you can adjust how quickly the idle circuit responds to changes. By increasing the size of the idle air-bleed, you can slow the response time and slightly lean the idle circuit for very fine adjustments to the idle fuel circuit. The same is true with the high-speed air-bleeds. Generally, working with the fuel side will generate quicker, more measurable results.
The HP Ultra billet metering blocks come with five interchangeable emulsion jets per circuit (making 20 overall for all four barrels) that create an almost limitless tuning arrangement for optimizing the main fuel delivery curve. Tuning these emulsion jets should be left to the experts, but the basic premise is that these emulsion jets introduce air into the main metering circuit. The amount of fuel in the main well is determined by the main jet. But this can be further trimmed throughout the entire fuel-delivery curve (from low engine speed to peak rpm) by adjusting the size of these emulsion holes. Making an emulsion jet larger adds more air and leans out a particular portion of the fuel curve. Conversely, reducing the size of the emulsion jet adds less air and richens the circuit. Air is introduced into successive emulsion holes from top to bottom uncovered as the float level drops during a sustained wide-open-throttle run such as at the dragstrip. From what we can gather from the carb tuners, these are the basics, but the circuits do not always respond in simplistic terms. Before the days of adjustable emulsion holes, this type of tuning required permanent modifications to the metering block. The beauty of aftermarket adjustable metering blocks is that the emulsion holes can now be easily tuned by replacing tiny jets. And if you get lost, you can always return to the original emulsion package and start over.
Circuits
There are more pathways for fuel through the carburetor than just the main metering circuit, so this is your opportunity to become the neighborhood Holley guru. All it takes is a little bit of study. The layout for HP carburetor metering blocks and Dominator carbs will look slightly different from this standard Holley two-circuit metering block, but most of the circuits are exactly the same.
(B) Idle downleg:
This passageway feeds fuel to both the idle discharge port and the idle transfer slot.
(C) Idle well:
Fuel from this well travels to the top of the metering block, then turns 180 degrees and mixes with air from the idle air-bleed into the circuit.
(D) Accelerator pump passage:
This transfers fuel from the accelerator pump to the outlet nozzle.
(E) High-speed air-bleed:
Air from the high-speed bleed enters the metering block here to be mixed with the fuel as it climbs the emulsion tube.
(F) Passage to booster:
This channel transfers fuel from the main well to the booster
(G) Ported vacuum passage:
This connects the ported vacuum source in the throttle body to the outlet where this can be routed to a source like vacuum advance.
(H) Parallel air well:
Air is introduced into the main well through these two holes.
(I) Main well:
Fuel collects here after passing through the main jet.
(J) Power valve channel:
This is where the power valve is located. The two small holes are the power valve channel restrictors (PVCR) that determine the amount of fuel added to the main metering circuit when the power valve opens. This valve determines when additional fuel is added to the main circuit.
(K) Idle restrictor channel:
Fuel from the main circuit passes through this short channel and through a small brass restrictor (L) that acts as the idle circuit jet.
(M) Idle transfer slot discharge:
Idle fuel exits the metering block to deliver fuel to the transfer slot.
(N) Idle fuel discharge port:
Idle fuel exits the metering block and enters the carburetor main body for carb idle fuel below the throttle blades.
(O) Dowel pin:
Two pins locate the metering block on the carburetor main body.
Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!