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Carburetor Secrets Exposed!

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by Huw Evans  More from Author

Choosing and Tuning Carburetors for your Hot Rod


When it comes to any hot rod or custom car, the engine is the lynchpin. Not only does it provide propulsion, it also defines the vehicle’s character. Whether a big cube V-8 or nimble in-line four, it will affect how the car accelerates and behaves. But crucial to making any kind of throttle response possible is the fuel system and how the juice is delivered into the engine’s combustion chambers. That’s where the carburetor or injectors come in. The merits of fuel injection are well known, but for the purpose of this article, we’re going to take a more traditional approach, focusing on carburetors for your hot rod – what’s available and how the right carb can make your car run at its very best.


A Simple Principle

Carburetors are very basic instruments, little more than a tube or pipe. At the bottom of the tube is a moveable flap (the throttle plate), while the pipe itself narrows below the throttle plate. At the narrowest point (the venturi), a jet is fixed at an opposed angle to the wall of the tube. Via a mechanical pump and reservoir (tank), fuel is fed by gravity into a chamber, which contains a float that controls the flow. Once in the chamber, the fuel is drawn into the tube through the jet. As it meets with the air, it is forced down the venturi and into the engine’s manifold and combustion chamber. The throttle plate controls the flow of air – the more open it is, the greater flow and the more combustible the air/fuel mixture is. This increases power, since the higher the volume of air, the more quickly it’s heated as it enters the chamber.

As automobile engines have to deal with different loads and throttle inputs to cover a wide variety of driving and temperature situations, carburetors generally have three cycles for delivering air and fuel to the combustion chamber, where it can be ignited. These are idle, part throttle and full throttle.

In an ideal world, the perfect air/fuel ratio for complete combustion, also known as the Stoichiometric ratio, is 14:1 (14 parts air to 1 of fuel), which maximizes combustion and minimizes exhaust emissions from the engine. However, in order to achieve anywhere near this 14:1 ideal, the carburetor and engine need to have reached optimum temperature.

At idle, the throttle plate is almost fully closed. That means there’s very little air pressure on the top, but a great deal of vacuum on the bottom since, velocity is sucking what air there is into the manifold. As a result, a secondary jet is located below the main jet, close to the bottom side of the throttle plate (the idle jet). This uses greater vacuum to draw more fuel into the intake manifold when the plate is closed.

When the engine is cold, it needs more fuel to reach the combustion chamber, since the air isn’t hot enough to achieve ignition from the sparkplug anywhere near the Stoichiometric ideal. The fuel needs to be forced down the tube faster to ignite the mixture. In order to make this possible, a secondary flap or valve, called a choke plate, is fitted under the main throttle plate. When the engine is cold, this choke completely shuts off airflow from the top of the carburetor, further increasing the vacuum in the bottom of the venturi and allowing a greater volume of fuel to flow into the manifold. As the engine warms up, the choke plate gradually opens, and by doing so reduces the flow of fuel from the idle jet as the air pressure decreases. Chokes can be manual, usually operated by a hand-pulled lever, or automatic, whereby they use an idle electrical circuit via a heat sensitive wire and element that’s linked to the vehicle’s ignition system and fuel cut-off valve. With an automatic choke, as the engine gets hotter, the wire element heats up and the choke plate opens, slowing the rate of fuel through the idle jet.

Once the choke plate is fully open at part throttle, the automatic choke is shut off, as fuel delivery is taken over by the main jet. The main jet or jets are usually specifically calibrated to deliver the required rate of fuel in relation to throttle plate position, using needle valves operated by screws on the side of the carburetor housing, which allow the fuel flow to be adjusted.

Nearly all carburetors use a mechanically-operated throttle linkage, either via rods and levers or cables to control the position of the plate, which controls the velocity of air and fuel, hence acceleration and power delivery of the engine. At full throttle, another jet, known as the air-bleeder, mounted above the main one, pre mixes the air and fuel via an emulsion tube and forces it down where it meets with the main jet and pushes the mixture into the venturi. This is designed to provide sufficient fuel flow that cools the air at higher temperatures. As the air heats up, it’s more at risk of igniting on its own under compression without a spark, which can cause flame outs in the combustion chamber, resulting in considerable engine damage.


Carburetor Design

The very first known carburetor, called the “Rotary-Brush Atomizer,” was created by Siegfried Marcus in 1875 for use on an internal combustion engine. It comprised of a reservoir or bowl that contained fuel and a rotary brush feeder, driven by a pulley. As the brush turned, it scooped fuel out of the reservoir and flung it into the air, where vacuum created by the engine’s pistons drew the mixture down into the engine. It was crude and not entirely effective. However, British engineer Fredrick Lanchester took the Marcus concept and refined it, adding wicks that ran into the reservoir from a compartment above. Fumes given off from both the wicks and fuel ignited once they entered the engine, promoting better combustion. However, like Marcus’ design, the Lanchester wick carburetor was plagued with the problem of regulated fuel delivery, meaning engine flooding was commonplace.

Enter Wilhelm Maybach. The famed German engineer developed an adaptation of the atomizer, which used a needle valve operated by a gravity-controlled float to regulate the flow of fuel. As gasoline entered the carburetor’s bowl, a float mounted in it would rise. When the float reached a certain level, it would activate the needle, which shut off the fuel flow. As a result, Maybach could fully control the amount of gasoline entering the engine, reducing the problem of flooding. His design was the grandfather of what we’ve come to know as the automobile carburetor.


Configuration

Maybach’s design was an updraft carburetor, i.e. the air and fuel were drawn up into the intake manifold. His added advantage was an oil bath air cleaner, in which a reservoir of oil droplets was drawn upward in a vacuum to coat a mesh plate, which helped prevent foreign objects from entering the air stream before the days of paper element air filters.

Since the early 1930s, when many cars coveted by hot rodders were first built, most US manufacturers have employed downdraft carburetors, where air is drawn in from the top and down into the intake manifold using a mechanical, camshaft-driven pump and gravity feed for the fuel delivery. Ford used single-barrel downdraft carburetors on its four-cylinder and early V-8 engines, but in 1934, they introduced a carburetor that has since become a legend in hot rod circles: the Stromberg 97. The 97 used twin barrels, which permitted a greater flow rate at both part- and wide-opened throttle. Strombergs eventually became so plentiful that they were the primary choice for a young group of speed merchants called hot rodders, who had a serious jones for going faster but not much money. Hot rodders discovered that one surefire way to making more power was utilizing multiple carburetors – the more tubes, the more flow and the more power available.

By the 1950s, they had made their own intake manifolds designed to accept multiple carbs. Three two-barrels were the most common, though some could install four or as many as six. However, advances in engine technology, especially higher compression ratios and better quality fuel, required fuel delivery methods that went beyond small diameter two-barrel carburetors. Furthermore, multiple carburetor setups could prove very difficult to tune, resulting in rough running issues that simply weren’t acceptable to the postwar motorist.

As a result, four-barrel carburetors, which used two primary barrels or tubes and two secondaries, became progressively more common during the 1950s for performance applications in passenger cars. These carburetors used two barrels and venturis during idle and part-throttle operation, using either a mechanical linkage or vacuum to open up the secondary barrels at wider-throttle openings.

Mechanical linkages use a rod to open the second set of throttle plates, and carbs equipped with them are often referred to as double pumpers, since they’ve got two accelerators. They were primarily conceived for use in high performance and highly modified engines, running lots of valve lift for high horsepower, requiring maximum air/fuel and very little vacuum. On street engines, geared primarily for low and mid-range power and torque, using engine speed to increase vacuum at higher rpms allows for the secondaries to open in a progressive fashion, lending less snappy, more temperate throttle response.

With four-barrel carburetors, you essentially got the best of both worlds – good idling, drivability and reasonable fuel economy (since all four barrels weren’t in operation all the time), plus progressive or all-out performance when you needed it, simply by punching the throttle. Examples of popular OE four-barrel carburetors included those offered by Holley, Carter and Rochester. Going a step further, some cars received dual four-barrel carburetors for maximum power, sometimes mounted diagonally on cross-ram manifolds to promote a greater spread of power and torque across a wider rpm range. Popular examples of dual quad setups include the Chrysler 413/426 Wedge engines, the 426 Hemi and Chevy’s Trans Am 302 V-8. Today four-barrels are widely available as used originals, or new from Holley, Carter and other aftermarket companies like Barry Grant and Edelbrock. No matter what engine you’re using in your hot rod and no matter what kind of intake manifold or air volumes required, there’s a carburetor to suit your needs and budget.


The Q-Jet

Among four-barrel carburetors, the Rochester Quadrajet deserves special mention. Unlike many other four-barrel units, which utilize venturis similar in size and configuration, the Q-Jet features fairly small primary barrels and much larger secondaries. This setup is designed to aid low speed drivability and fuel economy, while offering a major amount of additional wallop at wider throttle. The Q-Jet utilizes dual upper throttle plates for the secondary barrels, controlled by a secondary linkage from the primary throttle. Along with vacuum and metering rods that lift up as more throttle is applied, the flow of fuel from the jets in the secondaries is controlled in relation to the position of the valves. So when you punch the throttle, the plates open progressively, allowing power to build exponentially up until the engine’s maximum rpm, creating a serious amount of torque. Some of the quickest muscle cars and street machines of all time used big cube engines fed by Rochester Quadrajet carburetors.


Variable-Venturis

An interesting development in the world of carburetors was the concept of a variable venturi, in which the diameter of the main jet is controlled by a tapered needle mounted inside via a vacuum-operated piston. Among the most common types of variable venturi carburetors are SU and Hitachi side draft carburetors found on many European and Japanese cars of the 1960s and 1970s. Ford Motor Company also utilized a variable venturi, but it was a downdraft design with a fixed venturi barrel, which incorporated a moveable flap that provided a narrow venturi diameter at low rpm and a wider span at higher revs in an effort to maximize air/fuel at all engine speeds and reduce exhaust emissions. It was primarily used on 255, 302 and 351ci V-8s in the late 1970s and early 1980s, but proved problematic. V-V carburetors like this are seldom used today.


Tuning Tips

Given that carburetors are essentially low-tech devices, it might appear that tuning them for optimal performance and drivability is fairly straightforward. As with many things, what appears simple in theory isn’t always so in practice. Getting your hot rod or street machine to run at its best depends on many different variables, from the condition of the carburetor to the state of the car’s ignition system, ignition timing, frequency of use, quality of fuel, fuel filter, fuel pump condition – you name it. One of the most common problems for rough running vehicles concerns ignition timing. Advancing it can be a good way of increasing power, since you’re advancing the point at which the spark ignites the air/fuel mixture in the combustion chamber, before the piston reaches top dead center. However, to prevent detonation, the more you advance timing and raise compression in the cylinders, the greater the need for a slower- burning higher-octane fuel. Too much timing and too little octane can cause detonation. On many carbureter-equipped engines, ignition timing is controlled by mechanical spark and vacuum advance that can alter the timing depending on engine speed and load. If the engine has trouble breathing, especially at low rpm, advancing the timing is often considered one of the first remedies, but remember, a good running engine is only as strong as its weakest link.

Major running issues on older engines and older carburetors can often be attributed to stale fuel. On carburetor float bowls, fuel (which can break down in just 60 days), can leave behind a varnish, not only on the bowl, but also in the jets and bleeders, which can restrict fuel flow and make the engine hard to start or rough running, no matter how much air you’re forcing down to the intake. So when you’re at the stage of getting your hot rod running, make sure you thoroughly inspect and clean the main jets, idle jets and bleeders with carb cleaner, as they’re often overlooked – even by some professional carb rebuilders. This especially applies if you’ve bought used carburetors or engines via other enthusiasts or swap meets.

Emulsion tubes can also be a problem. If they’re too small, they aren’t able to properly mix the air/fuel before it enters the venturi. In many cases, utilizing slightly larger tubes with fewer holes for the air to escape can be an effective way of improving combustion and promoting smoother running.

Another tip is making sure the float level on the carburetor is set correctly. If it isn’t, your engine will be starving of fuel or flooding. Either way, this will cause rough running and potential damage, including cylinder wash and detonation. Many enthusiasts overlook the float as a potential fuel delivery issue. On some four-barrel carburetors, notably Carters and Rochester Quadrajets, people place too much emphasis on the secondary barrels, trying to get them to open faster. It’s fine if you’re building a race engine, but if you’re leaning toward street performance it will actually cause the engine to stumble, since there’s too much air flowing through the carb for the intake and cylinder heads to handle. Using the correct size primary metering rods goes a long way to making the carburetor run at its best, since it also helps regulate fuel delivery in relation to airflow.

Other issues can be a carburetor that’s too small or too large for the size of the engine, or a camshaft profile which is too mild or too aggressive and which can’t provide the required air volumes via valve operation needed for the carburetor. Other common problems can be the result of incorrectly set idle mixture screws (too rich or too lean) or even a faulty automatic choke that either shuts off too soon or stays on too long, which is often the result of engine cooling issues and a sticking thermostat.



Fluid and Flow

The Bernoulli Principle

How carburetors principally work is based on an equation devised by Swiss-Dutch Mathematician Daniel Bernoulli. Using the concept of conserving energy, it essentially states that the flow of mechanical energy in a fluid is constant at all points along the path of which it flows (streamline). If the fluid is flowing along a path from a source, the rate at which it flows is the same along the length of that path, because at that source, the sum of gravitational pressure is equal in all parts.

In its most basic form, Bernoulli’s equation can be depicted like this:

P + q = Po

Where P equals static pressure, q equals dynamic pressure and Po equals total pressure.

However, while the flow of fluid is constant, at each point along the path it has its own unique static pressure and dynamic pressure, allowing it to flow. So, when it comes to a carburetor venturi, this means that no matter what the flow rate of air may be, the rate of fuel in relation to it from the jets remains constant, creating a natural balance. This balance of air to fuel helps explain why seemingly random carburetors bolted on a random engine will still allow that engine to run, although not always that well. Think of street engines with oversized carburetors or vice versa as good examples which can often be found on any number of unfinished or discarded hot rod projects you come across in the classifieds.


CFM

When it comes to airflow, it pays to figure out the correct volume of air required for your particular engine application. Airflow in carburetors is measured in cubic feet per minute (CFM). Figuring out exactly what size of carb you need for your engine can be done by using this formula:

D x M / 3456 = CFM

D = displacement (cubic inches)

M = Maximum rpm

For example, if we had a 392 Hemi V-8 and wanted to calculate the required air volumes for our engine we would do:

392 x 5800 /3456 = 657.8 CFM

Bear in mind that this equation refers to an engine operating at 100 percent airflow or volumetric efficiency, which in practice is simply impossible. A more accurate assumption would be around 85 percent, since most engines operate at a volumetric efficiency level of between 80 and 88 percent. So, if we were to assume 85 percent of 657.8 CFM we get 559.13, or rounded up, 560 CFM for our carburetor.



Useful Sources:

Barry Grant
706-864-8544
www.barrygrant.com

Edelbrock
310-781-2222
www.edelbrock.com

Holley
270-781-9741
www.holley.com

Mr. Gasket
216-688-8300
www.mrgasket.com

Stromberg Carburetors
011-44-1473-811700
www.stromberg-carburetors.com






Downdraft carburetors were standard fitment on most Detroit-built cars from the early 1930s onwards. Among the most popular were Stromberg two barrels, first seen on Ford passenger car V-8s in 1934. Many early aftermarket carburetors were based on this design, including those offered by Holley.



 
Side draft carburetors, where the carbs are positioned horizontally to the engine, were primarily used on European cars, where engine bay space was at a premium. SU (for Skinners Union) two barrels were very popular on MGB Sports Cars. These incorporate a variable venturi that regulates the flow of fuel into the air stream to help deliver sufficient power across a wider rev range. Versions of these carburetors are still manufactured today.




Multi-carburetor setups started gaining in popularity in the 1950s. Most of the early ones were found on home-built hot rods, but automobile manufacturers were joining in by the end of the decade, as seen on this 348ci V-8 installed in a 1959 Chevy Impala.




Three two barrel carbs operated on the principle that during normal driving, the center carb mainly supplied the air and fuel mixture to the engine. Punching the throttle opened up the two outer carburetors – primarily by vacuum from the manifold – delivering a satisfying wallop of torque.



In this picture you can see the primary throttle linkage from the center two-barrel carb on this factory tri-power setup.



Holley four barrels are among the most popular carburetors with enthusiasts today. Cheap to buy and relatively simple to tune, they can be adapted for a wide variety of applications. This massive 780 cfm Holley is bolted on top of a Mazda rotary engine, whose high revving ability needs it to maximize flow from the large diameter barrels for peak horsepower.



Here you can see the primary carburetor barrels and the edge of the float bowl, which draws fuel into the carb venturi via jets built into the barrel.




A close-up of the throttle linkage reveals cable actuation, though mechanical rods are also often used.




Most four-barrel carburetors incorporate a primary and secondary linkage that can be entirely mechanically controlled (as on many high performance and race carbs), or via vacuum (as on many street applications).



Here you can see both the primary and secondary barrels on this Holley. Note that they’re equal in size.




Carburetors are very affordable, making them a good choice for a hot rod project. Sometimes you can even score a complete engine on the cheap, like this 1960s vintage Ford “Challenger” V-8.



Swap meet stuff like this can be good, because sometimes, along with the carb and engine, you can get your hands on vintage speed equipment, like this Offenhauser intake. Today, this equipment is often highly prized among rodders.



Mixture screws are used to set the air/fuel on carburetors and control the idle speed. They’re an integral part of tuning.




Sealing is an aspect of carburetor operation that’s often overlooked. A leaking manifold allows air to escape, reducing power and causing rough running. Choosing the right intake to carb gasket is crucial to give the air an unobstructed path into the combustion chamber.



Although smaller capacity barrels mean this carburetor flows less air volume than some high-cfm carbs, from a design aspect it’s almost identical.



Here’s a close up of the throttle linkage. Note that this is operated by a mechanical rod system.



As four-barrel carburetors go, one of the most interesting (and often misunderstood designs) is the Rochester Quadrajet, primarily offered on GM cars from 1965 through 1990.



Quadrajets feature a single upper throttle plate for the primary barrels and separate plates for the secondaries.



From underneath, you can see that the secondary venturis are much larger than the primaries. This is because the primaries were used mostly for normal driving and saving fuel, while the secondaries are designed to progressively open at higher rpm, delivering a real wallop of torque.



Original Quadrajets are stamped with this logo on the side of the carb housing. As emissions requirements became more important during the 1970s, the design of the carb, especially the shape of the metering rods, was altered to deal with lower compression ratios and retarded ignition.



Here you can see a close-up of the wire element and piston that provides automatic choke operation on the Q-Jet. As engine temperature increases, the choke plate is pulled back, reducing fuel flow from the idle jet located below the main one.



This image illustrates the float bowl and the fuel feed that leads to the carburetor jets. Fuel is fed into here from a cam-driven mechanical pump mounted on the engine block.



Compare the difference in venturi size between the Q-Jet, right, and an Edelbrock/Weber aftermarket four-barrel carburetor, left.


 
Weber carburetors are named after Italian engineer Edoardo Weber, who original offered them as part of a tuning kit on Fiats. Although primarily used on racecars from the 1950s to the 1970s, today’s Weber carbs can be tailored to all kinds of different applications, including street performance.



With both primary and secondary barrels open, you can see the shape of the venturis. The location of the throttle plates allows pressure to build up on one side, creating vacuum to draw fuel down into the engine.



Strombergs, particularly 97s which are being manufactured again, are becoming popular among those building traditional style hot rods. This one sports four of them in a row.



Getting your engine running right requires selecting the right combination. When this 1923 Model T hot rod was brought into a local performance shop, it was running rough, due to a large volume intake and carburetors that were too small.



Dual four barrels are a popular choice for many modern hot rod engines. Note the tunnel ram intake, designed to make maximum power at higher rpm.



Ensuring adequate fuel supply is crucial to ensure proper carb tuning and engine running. This setup sports braided lines for extra durability.



Using a fuel pressure regulator helps when tuning your carburetors, as it indicates the rate of fuel from the pump to the carb. Too little or too much fuel is a primary reason for rough running engines.



Carburetors use a mechanical pump to drag fuel into the float bowl and through the jets. Most mechanical setups operate at fairly low pressure – around 7-8 psi.



Many high performance Chrysler cars from the 1960s and early 1970s used mutiple carburetors, particular dual four-barrels. On modified street machines like this 1968 Dodge Charger that uses an aftermarket crate engine, it’s still a popular choice.



Edelbrock is among the most prolific of modern aftermarket carburetor vendors. Its line of Performer carbs feature aluminum construction to reduce cooling temperatures and reduce the risk of warping.



Because the Performers don’t use any gaskets below the float bowl, they’re also less prone to fuel leaks, which was often a problem on many vintage 1950s and 1960s carburetors.



Depending on the type of hot rod or street machine project you’re building, the right carb setup is crucial to helping deliver the required level of performance. This 1967 Plymouth GTX features an aftermarket Hemi V-8…



… with a high riser single plane intake for dual four barrel carburetors. The design and configuration of the intake are crucial to air flow, so you need to choose a carb that’s designed for high flow operation and top end power.



Intakes come in all shapes and sizes. Like carburetors themselves, used ones are often very affordable, and if you search hard enough you can be rewarded with some cool finds. Check out the vintage Edelbrock six pack intake on the left. Versions of this were used on the legendary Dodge Superbee 440 Six Pack and are known to help generate a serious amount of torque on a big cube street engine.



When it comes to carb tuning, patience and a set of quality tools go a long way to ensuring successful results.



Learning how to tune your carburetor well takes practice, and there are many variables, such as the condition of the fuel and ignition system, the quality of the gas, the condition of the carb and even the type of camshaft used.



Multiple carb setups are often more difficult to tune, but get it right and the results are well worth it.



Adjusting the valves is a crucial part in tuning a carbureted engine, since valve lift affects timing and combustion. This is especially true on cars equipped with solid lifters that need periodic adjustment to ensure proper running.



Advancing the ignition spark curve is an old hot rod trick used to coax more power by igniting the mixture before the piston reaches top dead center on the compression stroke. Modern timing lights have made the job much easier on older engines.



This 1967 Plymouth GTX utilizes dual four-barrels for maximum performance, in this case Edelbrock AFBs, which are the modern equivalent of the original Carters and manufactured by Weber USA.



Besides their performance value, multiple carburetor setups can also be thing of beauty, as seen on this custom, early 1950s Mercury.



In the 1960s and 1970s, it was popular to use massive double pumper four-barrel carburetors and a Roots style supercharger. These carbs were capable of supplying the required amount of fuel in forced induction applications to generate serious amounts of power in drag racing. This style is coming back into vogue, as witnessed on this tubbed and pro-streeted 1969 Mustang Mach 1, which sports a highly modified 351 Cleveland V-8.

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