In this world of digital control over nearly every facet of life, it might seem strange that the carburetor nation has never been stronger. The last Detroit production car with a carburetor was nearly 30 years ago, yet carburetors continue today as easy, simple fuel mixers that operate over an incredibly wide range of engine operation. A carburetor’s simplicity is its strength – you don’t need a laptop to tune it nor do you need a background in digital engineering to understand how it works. All you need to tune a Holley carburetor is a little knowledge, a nut driver, a Phillips screwdriver, and a clean work bench. We’ll lay out a few tidbits of Holley carburetor foundation knowledge in this story so that you can become the neighborhood Holley carb zen master.

Modular:

While Holley can trace its roots all the way back to the Holley brothers of Bradford, Pennsylvania in 1902, we’ll concentrate on the immensely popular performance four-barrel Holley that has become the dominant fuel mixer of the last 50 years. What made the Holley such a lasting icon is its modular design. Looking at a disassembled four barrel on the work bench, there are only six major components accompanied by a few simple support pieces that are also easy to remove, tune, or replace. Let’s start with some simple terms. Within the Holley four-barrel carburetor family, there are two versions that are differentiated by how the rearmost or secondary barrels are activated. The simplest of these is the mechanical secondary where a linkage opens the rear throttles once the primaries are roughly 60 percent open. The second is the vacuum secondary version. This carb employs a round vacuum diaphragm module that uses engine vacuum to trigger the opening of the secondary throttle blades. Most Holley four barrels are constructed of a zinc alloy but certain street and race versions are now available in aluminum that offer a measurable weight savings. A subcategory of the vacuum secondary carburetors (and some smaller mechanical versions) is the differentiation of 4150 and 4160 carburetors. This difference is also easy to spot as the 4150 version uses metering blocks located on both the primary and secondary sides of the carb between the carburetor’s main body and the float bowls. A 4160 style carburetor uses only a primary metering block, substituting a thinner metering plate that is hidden from view by the secondary float bowl. Among all the different carburetors on the market today, the mounting flange design is critical if only to ensure that the intake manifold will mate to a Holley carburetor. The Holley four barrel square mounting flange has come to be known as the standard flange. The other flange is the spread bore design championed by the Rochester Quadrajet carburetor. As the name implies, spreadbore primaries are very small accompanied by much larger secondaries spread wider apart, creating a unique mounting flange. Most other aftermarket carburetors follow Holley’s standard flange design. Another option for the Classic Holley is how fuel is introduced into the carburetor. Early and smaller cfm Holleys use a single fuel inlet configuration with a balance tube that directs fuel to the secondary float bowl. With increasing horsepower demands, Holley responded with the dual inlet where a separate fuel line is directed to each float bowl assembly. This is important when fuel demands exceed 450 horsepower because a small, singular needle and seat assembly can become a restriction as fuel flow increases. Within the Holley 4150/4160 family we can further split the family tree by separating the carburetors into intended areas of use. Traditional Holley street carbs feature a choke housing above the primary throttle bores and these are further subdivided into Street Avenger, Truck Avenger, and Double Pumper versions. With all of these carburetors, you have a choice of a mechanical or electric choke option as well as two different finishes. Off-road Truck Avenger carbs are easily identified by their distinctive vent tube that ties the primary and secondary portions of the carb together with multiple vent holes in the top. Double Pumper carburetors are Holley’s trade name for its line of traditional mechanical secondary carburetors that come in either the classic dichromate zinc or in a shiny zinc finish. The name originates from a term used by racers to describe an additional accelerator pump found on the secondary side which makes them easily identifiable. Originally offered only with a manual choke, Holley now supplies the Double Pumpers with electric chokes as well. As horsepower demands continued to escalate, one drag race modification that became extremely popular was to remove the choke housing to increase airflow. In 1994, Holley improved on that idea which led to the 4150 HP upgraded just recently to the Ultra XP line. These carburetors are intended as competition carburetors and are easily identified by their contoured radii that blend airflow into the venturis which also reduces inlet turbulence. The Ultra XP line of carburetors is the ultimate in 4150 style competition carburetors and is available in sizes ranging from 500 to 950 cfm. The pinnacle of high cfm rated carburetors exceeds the potential of the 4150 version carburetor. The Dominator line began back in the 1960’s intended for NASCAR racing but was quickly adopted in drag racing where carburetors and gasoline are king. The Dominator line features a larger mounting flange compared to the standard 4150 to accommodate huge 2.125-inch throttle plates that are airflow rated up to 1475 cfm.

Sizes:

Unlike Spandex and magic weight loss pills advertised on late night cable TV, when it comes to carburetors, one size most definitely does not fit all. Carburetors are commonly rated by cfm, which stands for the cubic feet per minute of air that each carburetor can flow. We won’t bore you with the complicated flow testing procedure, but it is essential to know that sizing the carburetor to the engine is important. For traditional street carburetors, the smorgasbord of carb choices is immense with sizes ranging from as small as 390 up to 850 cfm. Without getting into this subject too deeply, smaller engines respond better to smaller cfm carburetors. Small cfm carbs will deliver excellent throttle response on large displacement street engines but tend to limit peak horsepower due to restricted airflow. Taking an average 350ci street small-block as an example, Holley’s cfm calculator indicates that a mildly modified engine would demand a peak of 510 cfm of air at 6,000 rpm. While a 570 cfm Street Avenger carburetor would work just fine, a 600 or 650 cfm version would provide slightly more power potential for later engine modifications while not sacrificing throttle response or off-idle drivability.

Adjustments and Tuning:

One of the biggest advantages to the modular Holey design is its ease of adjustment. Among the aspects of engine operation that a carburetor controls, idle speed and air-fuel mixture are two of the most important. But beyond just these primary adjustments, most Holley carbs also allow the tuner to easily adjust the rate at which the secondary throttle plates open on a vacuum secondary carburetor. Other advantages include incredibly easy float height adjustment, something that cannot be said of other carburetor designs that demand the carburetor be partially disassembled to effect float height changes. Similarly, accelerator pump tuning is also much easier than other carburetors that may require machining to create the desired effect. The Holley carb’s modular design also makes tuning or repair easy because with four easily-removed bowl screws either the primary or secondary fuel bowls can be removed allowing access to the metering blocks for main jet or power valve tuning.

Conclusion:

There’s a simple reason the classic Holley four barrel carburetor has withstood both the test of time and technology to become predominant and influential fuel mixer over the last 50 years. It’s easy to work on, parts are inexpensive and plentiful, and the design has not changed significantly. The combination of simplicity of design combined with a tremendous range of choices make the Holley carburetor unique in the history of high performance. Holley four barrel carburetors come in an incredibly wide variety of styles which means there is a specific style of Holley that’s perfect for your performance application whether it’s a car, truck or boat. Let’s first take a look at the difference between a mechanical secondary Holley (left) and its street-friendly vacuum secondary brother (right). The arrow reveals the vacuum diaphragm module that uses engine vacuum to signal when the secondaries should open. This is completely tunable, making secondary opening virtually seamless. This closer view of the diaphragm housing on the passenger side of the carburetor shows the linkage that connects the diaphragm to the secondary throttle shaft. Holley Std. Flange (A) Spread Bore (B) Dominator (C) Not all carburetors use the same mounting flange. The Holley 4150/4160 square flange has become the industry standard (A) while some carburetors, like the Quadrajet use the spread bore style (B). Race engines requiring more cfm need a much larger Dominator 4500 series carburetors using a larger flange (C) that can flow up to 1,250 cfm. (X) (Y) Many Holley street carbs come with a single fuel inlet (X) while the competition demands for more fuel makes the dual inlet (Y) a definite requirement. Dual inlet carbs are also very popular if often just for that competition appearance. Street carburetors will be much more cold weather friendly when fitted with an electric choke mechanism that restricts inlet air until the engine creates sufficient heat to vaporize the fuel to idle properly with a leaner mixture. This disassembled view of the main components of a Holley carb reveal the main body (1), the throttle plate (2), the two main metering blocks (3), and the primary and secondary float bowls (4). Add a few small parts and some gaskets and that’s pretty much all there is to a four barrel Holley carburetor. Looking straight down the air’s pathway into the engine, fuel is introduced into the engine through the small round boosters located in the middle of each of the four venturis. Fuel exiting these boosters is quickly atomized and mixed with the air to feed those hungry cylinders. Holley now offers carburetors in shiny zinc (left) as well as the traditional zinc dichromate (right) finish. Many Holley models are now also offered in aluminum. Holley’s Street Avenger comes in either a shiny zinc or aluminum finish and can also be had in either electric or manual choke versions. All Street Avengers come with Holley’s new Gen IV No Trouble Power Valve with a million-mile blowout protection. The Double Pumper may be the most iconic carburetor in the performance world. These carbs now come with either manual or electric chokes and are all 100 percent wet flow tested with models ranging from 600 to 850 cfm. The upgrade from the Double Pumper is the Street HP. The cfm selection now extends from 650 through 950 cfm models available, and the 750 can be had with either vacuum or mechanical secondaries. The obvious change is the removal of the choke housing and the contoured venturi inlet. The Ultra XP line of carburetors is intended for high-output street and race engines. The upgraded carbs use billet aluminum base plates and metering blocks with multiple tuning options. The ultimate in high horsepower carburetion is the Dominator line with sizes ranging from 1050 to now a massive 1475 cfm. This passenger side view of the same Street Avenger carb shows the primary fuel inlet (1), idle mixture adjustment (2), electric choke (3), vacuum secondary diaphragm (4), and secondary fuel inlet (5).