What are the Differences in Big Holley 4150 Carburetors? Part 2

Wayne Scraba

17 Oct 2019

Click Here to Begin Slideshow We’re back with the second installment of the Holley Carb Comparo. Last issue, we took a look at cosmetics and basic construction along with the differences in boosters. This time around, we’ll dig into the float bowls, accelerator pumps and more. Check it out: The bowls all feature the same basic “cathedral” layout Holley has used since the Sixties. But the differences are many. Obviously, the construction differs – zinc for the Classic Double Pumper and the HP, aluminum for the Ultra XP. The Classic Double Pumper can be plumbed (inlet) on the passenger side only, while the HP and the Ultra XP can be plumbed from either side or both. The Classic Double Pumper and the HP have “Holley thread” inlets, while the Ultra XP is setup to accept AN fuel lines. Typically for Holley performance carburetors, they all have externally adjustable floats. The Classic Double Pumper and the HP have standard fuel level sight plugs in the bowls, but the Ultra XP is fitted with a clear fuel “window” on each bowl. One other big difference on the carbs is the float bowl drain found on the Ultra XP. It’s easy to access and makes draining float bowls easy. Finally, the XP fuel bowls are 20% larger than those found on the other two carburetors. Staying focused on the bowls, the HP and the Classic Double Pumper make use of a 30-cc front accelerator pump along with a 50-cc rear accelerator pump. The Ultra XP has 30-cc pumps front and rear. Moving to the side of the carburetors, there are considerable linkage differences between the Ultra XP and the other two carburetors. The Classic Double Pumper along with the HP use the same, traditional throttle linkage. But the Ultra features an all new-configuration with all of the street-oriented brackets and attachment points such as kick down brackets removed. This simplifies the carburetor dramatically in a high-performance application, but that’s not the end of it: Holley engineered an easily adjustable secondary linkage. The adjustable secondary link is manufactured from stainless steel and it can be set up as either a progressive secondary opening arrangement or as a 1:1 arrangement. The primary and secondary idle speed adjusters now have knurled knobs, which allow for easy hand adjustment (no tools necessary). Upstairs, the Classic Double Pumper has fixed air bleeds. Not so with the HP and the Ultra XP. They’re supplied with adjustable air bleeds. Here, the four outermost air bleeds are for the idle circuit, while the four inboard air bleeds are for the high-speed main circuit. The purpose of the high-speed air bleeds is to emulsify the fuel before it enters the discharge nozzle (where it is discharged into airstream in the venturi). As the air bleed size is increased, the air/fuel mixture leans out. When the size of the high-speed air bleed is decreased, pressure across the main jets will decrease. In turn, this pulls more fuel through the main system, which translates into a richer mixture. Holley states that the high-speed air bleeds act as an anti-siphon device. This prevents fuel from dribbling into the venturi when the airflow is reduced or stopped (throttle closed). Holley also points out that at high engine speeds, the air/fuel mixture must be on the rich side in order to prevent engine carnage (high speed lean out). When it comes to the idle air bleeds, keep in mind the idle system provides fuel at idle and low engine speeds. Under these conditions, the engine requires a richer fuel mixture than it does at high speed. With a lean idle mixture, combustion is slow and irregular. That results in a rough idle. Decreasing the size of the idle air bleed richens the mixture by increasing the pressure drop in the system. Increasing the size of the idle bleed leans the idle mixture by reducing the pressure drop across the idle air bleeds. Holley states you can accomplish the same thing by backing out the idle mixture screws, which will increase the pressure across the idle air bleeds, effectively pushing more fuel from the idle well. This creates a richer air/fuel ratio. Holley recommends you use the four idle mixture screws to adjust the mixture. See the accompanying slides for more details on the differences. And watch for our next segments. We’ll dig deeper into the big Holley 4150 carburetors. There really are considerable differences between the different models.

What are the Differences in Big Holley 4150 Carburetors? Part 2 — Click Here to Begin Slideshow We’re back with the second installment of the Holley Carb Comparo. Last issue, we took a look at cosmetics and basic construction along with the differences in boosters. This time around, we’ll dig into the float bowls, accelerator pumps and more. Check it out: The bowls all feature the same basic “cathedral” layout Holley has used since the Sixties. But the differences are many. Obviously, the construction differs – zinc for the Classic Double Pumper and the HP, aluminum for the Ultra XP. The Classic Double Pumper can be plumbed (inlet) on the passenger side only, while the HP and the Ultra XP can be plumbed from either side or both. The Classic Double Pumper and the HP have “Holley thread” inlets, while the Ultra XP is setup to accept AN fuel lines. Typically for Holley performance carburetors, they all have externally adjustable floats. The Classic Double Pumper and the HP have standard fuel level sight plugs in the bowls, but the Ultra XP is fitted with a clear fuel “window” on each bowl. One other big difference on the carbs is the float bowl drain found on the Ultra XP. It’s easy to access and makes draining float bowls easy. Finally, the XP fuel bowls are 20% larger than those found on the other two carburetors. Staying focused on the bowls, the HP and the Classic Double Pumper make use of a 30-cc front accelerator pump along with a 50-cc rear accelerator pump. The Ultra XP has 30-cc pumps front and rear. Moving to the side of the carburetors, there are considerable linkage differences between the Ultra XP and the other two carburetors. The Classic Double Pumper along with the HP use the same, traditional throttle linkage. But the Ultra features an all new-configuration with all of the street-oriented brackets and attachment points such as kick down brackets removed. This simplifies the carburetor dramatically in a high-performance application, but that’s not the end of it: Holley engineered an easily adjustable secondary linkage. The adjustable secondary link is manufactured from stainless steel and it can be set up as either a progressive secondary opening arrangement or as a 1:1 arrangement. The primary and secondary idle speed adjusters now have knurled knobs, which allow for easy hand adjustment (no tools necessary). Upstairs, the Classic Double Pumper has fixed air bleeds. Not so with the HP and the Ultra XP. They’re supplied with adjustable air bleeds. Here, the four outermost air bleeds are for the idle circuit, while the four inboard air bleeds are for the high-speed main circuit. The purpose of the high-speed air bleeds is to emulsify the fuel before it enters the discharge nozzle (where it is discharged into airstream in the venturi). As the air bleed size is increased, the air/fuel mixture leans out. When the size of the high-speed air bleed is decreased, pressure across the main jets will decrease. In turn, this pulls more fuel through the main system, which translates into a richer mixture. Holley states that the high-speed air bleeds act as an anti-siphon device. This prevents fuel from dribbling into the venturi when the airflow is reduced or stopped (throttle closed). Holley also points out that at high engine speeds, the air/fuel mixture must be on the rich side in order to prevent engine carnage (high speed lean out). When it comes to the idle air bleeds, keep in mind the idle system provides fuel at idle and low engine speeds. Under these conditions, the engine requires a richer fuel mixture than it does at high speed. With a lean idle mixture, combustion is slow and irregular. That results in a rough idle. Decreasing the size of the idle air bleed richens the mixture by increasing the pressure drop in the system. Increasing the size of the idle bleed leans the idle mixture by reducing the pressure drop across the idle air bleeds. Holley states you can accomplish the same thing by backing out the idle mixture screws, which will increase the pressure across the idle air bleeds, effectively pushing more fuel from the idle well. This creates a richer air/fuel ratio. Holley recommends you use the four idle mixture screws to adjust the mixture. See the accompanying slides for more details on the differences. And watch for our next segments. We’ll dig deeper into the big Holley 4150 carburetors. There really are considerable differences between the different models.

All three carbs have traditional style Holley “cathedral” bowls. On the Classic Double Pumper, plumbing is restricted to one side of the carb (the passenger side).

Meanwhile, the HP series carburetor uses a Dominator style bowl. It can be plumbed from either or both sides. The HP and the Classic Double Pumper use a conventional Holley thread on the fuel bowl fittings.

Here’s a look at the Ultra bowl. It makes use of AN fittings. Additionally, the bowl capacity is now 20% larger.

The Ultra XP has a see-through float level window.

The others make use of conventional sight plugs.

Holley added a simple, easy to access bowl drain on each of the fuel bowls on the XP-series. Obviously, these carbs are racer-friendly.

Here’s a look at the linkage arrangements on all three carburetors. The earlier carbs have similar linkage, based upon street style applications while the XP uses a racer friendly system.

On the secondary side, the XP has an easy to adjust system and it can be setup to go from progressive all the way to 1:1.

On the Ultra series carb, the idle speed screw for the secondary throttle blades can be set by way of a screwdriver, but as you can see, the knobs are knurled. That means you can set idle speed quickly by hand.

Air bleeds on the 4781 Classic Double Pumper are fixed.

Meanwhile, the HP series carbs are designed with adjustable air bleeds for both the idle and high-speed circuits.

The Ultra XP is as well. See the text for more info.

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