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Carburetors Are Still Cool Especially the Latest FST Carbs

Hot rodders are traditionalists. While the latest fashions flash across the landscape and then fade to black, certain items remain unfettered by the passage of time. As long as there are hot rods, red 1932 roadsters, Flathead engines carburetors will never fall out of grace. The promise of precise fuel control with EFI continues to be attractive, but theres something about a carburetor thats essential and pure. So its slightly surprising that with all the attention lavished on digital devices that someone would be willing to advance a brand-new carburetor line at the time when EFI seems almost preordained. Yet here it is, FST Carburetors is introducing a line of four-barrel carburetors while the rest of the world focuses on self-driving cars and artificial intelligence. And hot rodders like us will rejoice, if for no other reason than these are all designed and built in a suburb of Detroit. The FST Carburetors line of carburetors extends from the basic, entry-level, 600-cfm vacuum secondary carb to the more exotic mechanical carbs. On the left is a cast body RT-X version 650-cfm mechanical secondary version. The black Billet X-treme is a 750-cfm mechanical secondary while the center version is a 1,150-cfm Billet Excess Pro model. FST is not reinventing the wheel. In fact, if these new carburetors look very much like a traditional square-bore carburetor thats because they are. Supporting that concept is that if youve got a stash of modular carb jets, gaskets, accelerator pump nozzles, and other small tuning parts they will all interchange. But the real push is in the materials and how these carburetors are built. FST is taking an aggressive stance. The product lineup includes eight separate lines of carburetors. The first fourRT, RT Plus, RTX, and RTX Viperfeature a vacuum-cast body with pressure-forged metering blocks and throttle bases. These are available in either vacuum or mechanical secondary versions. While the halo side of the FST line is the higher-end carburetors, FST hasnt forgotten the little guy. The affordable line of RT carbs starts with this 600-cfm vacuum secondary, cast body carburetor with an electric choke. The RT ranges in cfm ratings from 600-750 cfm in either vacuum or mechanical secondary configurations. The upper tier lineup includes the Billet X-treme, Billet X-treme Pro, Billet Excess, and Billet Excess Pro that are pure billet carbs. These versions are for the high-end user and racer that incorporate billet metering blocks with six stages of emulsion tuning and can be optioned with either two or three circuitsand theres even a custom four-circuit carb available. These are all mechanical secondary carbs but with lots of features that are most often only found on custom-built carburetors. The cfm range of the FST lineup spans the logical sizes from entry-level street 600-cfm cruisers to the Billet Excess race versions of the 4500-style air movers that can flow up to 1,250 cfm. FST even offers a custom version that can push that number to a massive 1,700 cfm, should that become necessary. All FST carburetors are quality tested on an actual running small-block engine before they are delivered. This ensures there are no leaks and that all the circuits are working properly. This is a Billet X-treme 750-cfm carburetor, which is really intended for race applications but can be used on the street. Lets start with the entry-level carbs and why FST delivers a little more than you might expect. The base-level RT carbs are available with either side hung or the more performance-oriented center-hung floats. The center-hung bowl versions also come with four-corner idle adjustability while the side-hung bowl carbs are more affordable and are fitted with two idle mixture adjusters. Its not a small thing that every FST carb comes with stainless steel throttle shafts and linkage components with some interesting and useful upgrades. Attractively, FST offers a 600-cfm carburetor with two-hole idle circuits, an electric choke, side-hung bowls for under $300 from Summit Racing, and a 750-cfm version (PN 41750) in either mechanical or vacuum secondary with four idle circuits for only slightly more. These are not cheap, offshore knockoffs. One look at the brushed vibratory polish or the black electroplated finishes reveals the care taken with these units. All FST carburetors, regardless of cfm or configuration, are fitted with drop-leg boosters. These boosters are more active than straight-leg boosters because the booster is positioned closer to the venturi. So how good are these fuel mixers? Well run through some of the features and sizes and then you can decide. Often its the little things that make a difference. Carb geeks will tell you that the booster design can have a big effect on driveability and throttle response. Dropped leg booster first gained notoriety on the 60s Holley 780-cfm carbs. Budget carbs are generally fitted with a less responsive straight-leg booster. All FST carbs, even the entry-level, 600-cfm, side-hung carb use drop-leg boosters. Other important features are four-corner idle circuits on most carburetors that can contribute to a stable idle, especially when used on single-plane intake manifolds and engines with big cams. Other small items we noticed were the correct linkage holes for positioning the TV cable for GM 700-R4 and 200-4R automatic overdrive transmissions. Most other carburetors require a bolt-on bracket. A big plus for serious carb tuners is that even the most basic carbs come with screw-in idle and high-speed bleeds on top of the carb, and fully adjustable screw-in bleeds in the metering block. These bleeds make it much easier to fine-tune a carb to the engines specific needs. All FST carbs, with the exception of the base RT model carbs, come with a four-corner idle circuit, as shown here, with this blacked-out 750-cfm Billet X-treme carb. This helps with idle stability on engines with big camshafts and single-plane intake manifolds. As an example, most performance carburetors are fitted with a compromise idle feed restrictor. This is a small restrictor in the metering block that establishes how much fuel is available to the idle mixture screws and to the transfer slot for smooth idle and off-idle driveability. The best way to ensure a smooth-running engine right out of the box is to fit the carb with a larger idle feed restrictor. Since the manufacturer has no idea of how its carburetor will be usedlike on a mild 350 or on a big-cammed 454 big-blockits always best to err on the rich side. But if your engine is milder, likely it could benefit from a leaner idle feed restrictor. With screw-in bleeds its very easy to measure the idle feed restrictor size and install a slightly smaller bleed. For example, if the stock idle feed restrictor is 0.035 inch, installing a 0.033-inch jet might help in a number of areas since it will offer more adjustability range to the idle mixture screws. This is just one minor example but there are dozens of others available to those willing to learn how to tune their carburetor for their engine. Setting the primary throttle blade placement for idle speed on big-cam engines can be customized with slight changes on the secondary side. FST makes this easy with this easily accessed adjuster screw. Also note the small cutouts in the metering block that offers a convenient leverage spot to help remove the metering blocks when the gaskets stick. Among the trick items built into each of the billet carbs starting with the Billet X-treme is a slick idle bypass feature. Engines with big cams that idle below 10 inches of manifold vacuum require more throttle opening, which exposes too much of the idle transfer slot. This is generally not a good thing because it causes the engine to suffer a flat spot off idle. One trick is to slightly open up the secondary idle stop, which allows closing the primary throttle a similar amount. On most of FSTs mechanical secondary carbs, the secondary idle stop is adjustable without having to turn the carburetor upside down. The adjustability doesnt stop there. The billet carbs also offer easily accessible idle bypass openings in the top of the carb. The four small brass restrictors can either be drilled or removed to allow more air into the engine, which allows the tuner to return the primary throttle plates to their proper position relative to the idle transfer slot. Previously, the classic solution to allow more bypass air was to drill permanent holes in the throttle blades. This was acceptable but then made it difficult to use the carburetor on a milder engine because the idle speed would be excessive unless the holes were filled. This idle bypass feature is incredibly useful and allows the carburetor to be used on other engines merely by reinstalling the screw-in plugs. The Billet series of carburetors offers these small restrictors that can be drilled or removed to create an idle bypass for big-cam engines. This allows retaining the proper primary and secondary throttle blade positions and should eliminate the need to drill holes in the throttle blades to achieve this position. We should also mention that most street carburetors normally come with two small air bleeds at the top of each venturi. The outboard one is for the idle circuit while the inboard version is called a high-speed. These circuits introduce air into the idle or main wells and because the bleeds are changeable, they can be adjusted for minor tuning effects. FST has expanded on this concept and added three and even four-circuit versions. These would be strictly race applications but they do offer the tuning expert the opportunity to make minor changes to produce a finely tuned fuel curve for a specific engine. The billet carbs also employ a billet fuel bowl that offers an option to feed fuel from either side of the bowl and a Viper bowl option that offers a third inlet in the center of the bowl if that captures your fancy. With a widespread offering of cfm sizes, mechanical or vacuum secondary options, along with other more specific tuning options, FST has a string of carburetors in which one is likely an ideal fit for your hot rod. For true believers, the carburetor is far from dead and these new fuel mixers from FST underscore that fact. This may be the 21st century but theres still room for a little bit of style from good analog fuel mixer even in todays digitally fixated world. SRM The FST Billet line features a built-in fixture that allows mounting a dedicated throttle position sensor (TPS). This is especially useful for vehicles using an electronically controlled overdrive automatic trans behind a carbureted engine. This can also be used in a road race situation to data log throttle position on the track. Usually reserved for serious drag race engines, the Billet Excess Pro Line pushes the envelope of ultimate tunability with two-, three-, and four-circuit options and passages intended to manage the entire range of fuel from gasoline, ethanol, and even methanol. The baseplate offers both 4500 and 4700 bolt patterns. All the FST billet metering blocks are drilled and tapped for screw-in air and fuel restrictors. This makes changing items like the idle feed restrictor (arrow) much easier. This does require a collection of small bleed restrictors but it allows very small changes that can have a major effect on throttle response and efficiency. This is a billet metering block with five emulsion holes; street carbs use four. Not only do all FST carbs use stainless steel throttle linkage with Teflon seals but the company has also added the extended location off the linkage to mount the stud for the throttle valve (TV) cable for mechanical overdrive transmissions like the GM 200-4R and 700-R4. Also note that the main throttle linkage hole is 1/4 inch for ball studs or bolts so the reducer adapter is no longer necessary. All street carburetors come with the linkage pieces that can be added to allow use of the older Ford automatic transmission kickdown linkage. This is a view looking down on a Billet X-treme Pro 750. All cast body street carburetors use two circuits: one for the idle circuit and a second for high-speed operation. This carb is a three-circuit but FST also offers an option on one of the larger carburetors for four circuits, encompassing idle, intermediate, lower high-speed, and upper high-speed circuits. In this case, the inboard bleed (closest to the vent tube) is the high speed, the middle is the idle bleed with the intermediate bleed on the outside. The post Carburetors Are Still Cool Especially the Latest FST Carbs appeared first on Hot Rod Network .

http://www.hotrod.com/articles/carburetors-are-still-cool-especially-the-latest-fst-carbs/

 

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