Holley's Ultra XP 4150 Is One Trick Small-Bore Race Carburetor

If you’re into motorsports in general and racing in particular, you’ll know that the standard for carburetors has always been Holley’s 4150 double pumper series. Period. There's a good reason for this: They’re easy to overhaul. Parts are plentiful and easy to find. Best of all, they’re incredibly easy to tune. Today, even with big swings toward EFI systems, plenty of racers and enthusiasts still rely upon the venerable 4150 to get the job done. Over the years, Holley has performed numerous upgrades on the carburetors (no secret). But the most recent was the biggest and it proved to be an extremely comprehensive overhaul. Racing technology marched forward and so did the Holley double pumper. To get there, the engineers from Holley took the 4150 racing lineup and pretty much rebuilt it from scratch.

Included in the mix are 600, 650, 750, 850 and 950 CFM examples. They’re available with calibrations for both drag racing, circle track and marine applications. Several examples are calibrated for methanol or E85. You can specify them in “hard core gray”, tumbled (shiny) with red billet components and tumbled (shiny) with black billet components.

So what’s inside the newest 4150: Take one look and it becomes rather clear something is radically different. The Ultra HP 4150 isn’t the same old same old with a few modifications. Much of the carburetor is manufactured from aluminum rather than die cast zinc. Weight reduction alone is impressive (roughly 38% lighter than the predecessors). Externally, the carburetor includes a baseplate machined from 6061 billet aluminum. This provides for superior gasket sealing on both the carb body-to-baseplate along with the baseplate-to-intake manifold. Note too, the elongated mounting holes in the base plate. What’s up with that? The slotted mount holes allow the carb to mount on a standard 4150-series intake or to mount on an intake manifold designed to accept a 4500-series Dominator (no adapter required). Furthermore, the baseplate incorporates a boss that can be opened up to provide for a vacuum source. Another benefit of a billet baseplate is the fact it’s very sturdy. In the case of older carburetors, it’s not uncommon to come across a cracked or broken baseplate (the cause is usually over-exuberant tightening – regularly coupled with tools way too big for the job). Using billet for the baseplate simply creates a more robust component.

On the side of the carburetor, you’ll find the throttle lever is an all new-configuration. All of the street-oriented brackets and attachment points (for example, the kick down bracket) have been eliminated. What this does is simplify the carburetor in a high performance or race application. Another great feature is the easily adjustable secondary linkage system. The adjustable secondary link is manufactured from stainless steel and it can be set up to provide a progressive secondary opening arrangement or it can provide a 1:1 arrangement. Also note the primary and secondary idle speed adjusters are now fitted with knurled knobs. This allows for easy hand adjustment (no tools required).

Dig deeper and you’ll find the throttle shaft has been slabbed and undercut to allow for improved airflow. It’s a small thing, but the primary throttle shaft is now capped which positively prevents vacuum leaks. A machined accessory-mounting hole is included, and this allows for easy addition of an adjustable primary throttle stop. There’s also a new secondary pump cam lever engineered to provide for a positive throttle stop on the secondary side.

The billet metering blocks are designed for maximum adjustment capacity and feature eight emulsion bleeds on each side of the power valve, replaceable idle feed restrictors, and power valve channel restrictors. The fuel bowls can be fed from either side and feature large, clear windows on each side of the bowl, a "trough" that directs fuel directly to the jets, and notched floats front and rear, which allow you to use a set of jet extensions, which come with the carburetor. There's also a racer-requested addition: bowl drains.

Holley Engineers weren’t quite done with billet aluminum parts. Both metering blocks are machined from 6061 aluminum. When you ponder the wear and tear a set of metering blocks go through (particularly in a racing environment) stepping up to billet parts just makes sense. Sealing is obviously improved too. A handy pry slot on each meter block has been added to allow for easier disassembly.

That’s not the end of it: The fuel bowl capacity has been increased by 20%. The added volume reduces the chance of fuel starvation. Taking a closer look at the bowls you’ll see there’s now a special “shelf” located beneath the needle and seat. The idea here is to minimize fuel aeration. A trough has been engineered into the bowl to direct fuel to the jets. This helps to keep them covered, which in turn, stabilizes the air fuel ratios. Further to this, the bowls are internally baffled. Fuel slosh is minimized when you get on the throttle. Externally, the bowls are now equipped with see-through windows for easy float settings. Fuel bowl inlets are machined in 8AN (o-ringed). This change allows for a wide range of plumbing options, and similar to Dominator carburetors, the bowls are threaded and tapped on either side. One end of each bowl includes a -6 AN to -8AN fitting while AN plugs are included on the opposite side. One incredibly useful new feature is easy-to-access drain port on each bowl.

Holley has included an integrated idle bypass valve. You’ll probably be wondering what this is all about. The bypass valve is similar to what you might find in many fuel injection applications. In a fuel injection system, the idle bypass is there to maintain airflow while the throttle is closed. With older carburetors, this was accomplished by either drilling holes in the throttle plates or by adjusting the curb idle speed screw(s). If a radical (big duration and/or big overlap) cam was used, one could get into trouble by opening the throttle too much, fully exposing the transfer slot. That doesn’t happen here. The idle bypass is pre-set but it can be easily fine-tuned by way of a simple screwdriver to compensate for a lumpy camshaft. In truth, it’s recommended the idle bypass is set before the curb idle speed screws are touched.

The idle system provides fuel at idle and low engine speeds. No secret. When idling, the engine requires a richer fuel mixture than it does at high speed. If the idle mixture is lean, combustion becomes slow and irregular. This results in a rough idle. Decreasing the size of the idle air bleed enrichens the mixture by increasing the pressure drop in the system. In contrast, increasing the size of the idle bleed leans the idle mixture by reducing the pressure drop across the idle air bleeds. It’s possible to accomplish the same thing by backing out the idle mixture screws. This will increase the pressure across the idle air bleeds, effectively pushing more fuel from the idle well. As a result, it creates a richer air/fuel ratio. It's recommended (at least for the average enthusiast) you use the four idle mixture screws for adjustment rather than tuning by way of the idle air bleeds.

The Ultra XP line features replaceable air bleeds for the idle and main circuits (the inboard bleeds are for the main circuit). A lot of design effort went into improving airflow within the main body of the carburetor. The boosters on this 650 CFM example are down-leg style, which provides an improved fuel signal. The adjustment for the idle bypass system is hidden underneath the air cleaner stud. This will help engines with larger camshafts that wouldn't ordinarily like to idle. Even the accelerator pump shooter screws saw attention...they are now smooth, round button-head examples.

With the metering blocks removed from the carburetor, you’ll discover they’re tunable for just about anything. Aside from the standard replaceable jets and power valves, the metering blocks allow for tuning the idle feed restrictions, emulsion bleed restrictors and the power valve channels. The idle circuit calibration is determined 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. 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. Given the new design of the metering block, it is possible to change the idle feed restrictor size in order to compensate for a large (big duration, high overlap) camshaft. With a combination such as this, the intake charge is often diluted at idle. The dilution is created because the intake charge is pulled out the header due to a late closing exhaust valve. To bring back idle quality (and sensitivity) to the idle mixture screws, the idle feed restriction size must be increased.

Power valve channel restrictions are included in each metering block. There are two restrictions, visible when the power valve is removed. These restrictions meter the flow of fuel into the main carburetor well. The diameter of these restrictions dictate 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 have no effect (obviously).

Ten emulsion bleeds are included on each metering block. Emulsion bleeds can have an effect upon power, but this is very dependent upon the engine combination. A word of caution here: Emulsion bleed tuning is accomplished on a dynamometer where it is possible to carefully monitor the air/fuel ratio. Emulsion bleed tuning components are available, ranging from a twenty-bleed kit to emulsion bleed two packs.

The bottom line here is, it’s clear the latest 4150 Ultra HP series of carburetors have been engineered and constructed with the racer and high performance enthusiast in mind. They provide performance adjustability never seen before in carburetors. The truth is, EFI might be hot, but there’s still a lot of life left in the 4150.

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