A look Inside FST’s Ground-Up Redesign Of The Racing Carburetor

Racing and performance carburetors have gone through many generational upgrades over recent decades, beginning with modifications to Quadrajet, Carter, and the most venerable Holley designs. Today’s versions of elite carbs are designed from scratch.

These new generation carburetors contain the common denominator of higher quality materials, ultra high accuracy machining, and overall design changes that stem from many years of working on the carbs of years past.

The FST Billet X-treme Pro Series Carburetor may be a “looker” with its billet and hard-anodized aluminum construction, but it’s the workmanship inside that provides exceptional tuning adjustments.

Beginning with what used to be called a clean sheet of paper, FST Carburetors combined the braintrust of its many team members with vast experience from the carburetor world. First-class designs with CAD software and high-end CNC machining processes don’t just automatically produce an improved carburetor; it takes the knowledge of which design changes are needed to benefit from clean-slate engineering.

We were provided one of the new Billet X-treme Pro Series carburetors to look over, dissect, and discuss with Scooter Vester, Manager at FST Carburetors.

Notable features that jumped out at first observation are the notches machined onto the metering blocks. Inserting a small screwdriver in these notches helps separate the blocks from the main body without damaging any of the adjoining faces. The air bleed jets are recessed, which protects them within the body design. These are examples of the experience by FST staff, who created these new features based on years of hands-on experience with designs of the past.

“Everything is billet,” Vester said. “The 4150 series carbs start at 600 cfm and come with a polished finish to the aluminum. The 4150 series design is offered all the way up to up to 1,200 cfm. These different series of carburetors are broken down in two different design formats. We have the Excess design, which is more of a performance/street carb, and we’ve got the Excess Pro. The Excess Pro models are broken down into two- and three-circuit metering block designs. All of the metering blocks are engineered with six stages of emulsion. This emulsion circuit design offers a wide range of tune-ability.

“Our carbs are the culmination of many years of modifying and custom designing everything from individual components to an entire carburetor design,” Vester added. “The bottom line is that we all got together and combined our real-world experience and didn’t just reinvent the wheel, but tried to make the very best wheel that lives up to today’s standards.”

Here is a short list of features included with the unit we’re looking over: billet aluminum materials, a three-circuit metering block, mechanical secondaries, and four-corner idle circuitry.

“A lot of credit goes to Philip Freeland; he designed 90-percent of the carburetor based on our staff input,” Vester explained. “The Viper float bowl design is Freeland’s effort that began from scratch.”


Even though individual components will interchange with many modular 4150 or 4500 carb designs, we really thought outside of the box about how each FST component is engineered and operates. – Scooter Vester, FST

The FST carburetor lines begin with the 600-CFM RT-series, which is more of a street series carburetor offering varied vacuum and electric choke options. There are 36 models to match your exact needs, each with different float bowl, choke, and secondary circuit designs. The racing-oriented 4150-models go up to 1,050 cfm, with no less than 19 models available.

“We have so many different series and specific models of carburetors to match the needs of so many particular street and racing applications,” explained Vester. “I can’t implore enough to call us and describe in detail your engine and car combination; we can be a big help in recommending exactly what FST carburetor will match your needs.”

The FST 4150 street (with choke) carbs range from 600- to 850 cfm in size. The all-out performance 4150 models (without choke) include the RTX and Billet Extreme designs. If the largest 4500 design carbs are what you need, the Billet Excess models range from a 1,050- to a massive 1,450 cfm.

“Stainless steel components are another contributing factor to the quality,” Vester said. “The butterflies, sealed throttle shafts, and fasteners are all stainless.”

Job one by FST is to design a carburetor with the broadest range of tuning options. The internal passages of the body, metering block, and fuel bowls are ways they accomplish those goals.

With that said, the FST group is not afraid to distinguish and describe in their materials when a proven design is implemented in their carbs in place of a redesign.

Between the FST main body and metering blocks, there are five stages of emulsion that offer a more extensive range of tunability compared to older designs. The emulsion system is used to blend air with fuel before it is introduced into the carb's venturi.

“The booster systems are just a standard design with no tricks,” said Vester. “We’re not afraid to say when something’s been proven and works well. We are not into redesigning something just to make it different. There are plenty of upgrades included that are our own design we want to concentrate on.”

“We did raise the booster within the fuel bowl and made the venturi a little longer,” Vester explained. “This gives the booster circuit a better airspeed and makes it more responsive to tuning.”

Our example carb is a smaller 4150-size unit measuring out at 1,050 cfm. Flow numbers for the unit rival the physically larger 4500-series carburetor models. The list of features continues, such as hardware, throttle shafts, and butterflies all made from stainless steel. Even the throttle levers and accelerator pumps appear constructed of heavier-wall material.

Within the metering blocks, the three-circuit design is what provides the FST carburetor its broad adjustability. Between the air bleeds and the bowl, additional circuitry is included that basically gives you more fuel/air control. Some of their street and performance models still use a two-circuit design. Vester notes that a two-circuit carburetor is much easier to tune for a street application.

This circuit is often described as a high-speed air-bleed. Offering this emulsion circuit with more stages of emulsion provides greater adjustability in a wide-open throttle application.

The Viper three-circuit fuel bowls have a higher capacity, as well. The bowls utilize a center-hung float design along with fuel level sight glass on both sides of each bowl for easy tuning without leaking fuel from an open orifice.

Pre-machined into the carburetor body are boost reference ports that can be used by simply drilling out the pre-machined boss. Our Billet X-treme Pro Series Carburetor also has a mount designed at the base plate to add a General Motors throttle position sensor if needed.

“We also have engineered options such as provisions for a boost reference port that are built-in,” added Vester. “On the passenger side of the carburetor, you can drill through a pre-threaded boss and have a threaded port for boost reference. This port also keeps the boost reference holes away from the throttle linkage. That is a new design we developed that racers are loving.”

If you want to bring your existing 4150- or 4500-series up to higher specs, FST is able to offer a vast selection of its unique components for sale separately from the complete carb lines.

“Basically, almost every single metering block, fuel bowl, and main body is interchangeable with – for example – Holley, Quick Fuel, or APD carburetors,” said Vester. “We thought outside of the box when it came to our engineering, but the components are still modular in design, which means they fit other carbs across the board.”

The bowls also have enlarged threaded access points that allow you to swap jets without removing the float bowls from the carburetor. Hardware that was once added for all-out competition, such as jet extensions and leak-free fuel bowl level sight glasses are all included right out of the FST Carburetor box.

Every chance to offer greater tuning parameters in a carburetor expands the effectiveness of your entire racing or performance engine. These next-generation carburetors from FST will provide finer tuning options, but also a broader range of circuitry to react to your needed air and fuel flow.

To recite the adage that “more is better” sometimes is applicable, sometimes not. When it comes to these increased metering, emulsion, and air bleed designs, more is good.

Article Sources

About the author

Todd Silvey

Todd has been a hardcore drag racing journalist since 1987. He is constantly on both sides of the guardwall from racing photography and editorship to drag racing cars of every shape and class.
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