Carburetor Basic Training

March 3rd, 2011

Text and Photos by Dave Emanuel

Chapter One of a Three Part Series

If you’re new to drag racing, it may seem odd that some engines are equipped with carburetors. If you’ve been racing for years, it probably seems odd that some aren’t. In either case, carburetor operation may be a bit of a mystery.

Back in the days when drag racing was as much about racing from traffic light to traffic light as it was about quarter-mile times, a mechanic didn’t need a great deal of expertise to be considered a “carburetor expert.” Carburetors ruled the engine compartments of virtually every vehicle available, and Holley and Carter four-barrels ruled in high performance venues. A good deal of mystique has always surrounded four-barrel carbs, but truly knowledgeable mechanics were usually easy to find. Since that time, electronic fuel injection has replaced carburetion on new vehicles (some people have never owned a vehicle that was equipped with a carburetor) and with each passing year, information and advice concerning all aspects of carburetor operation become increasingly more difficult to find. At the same time, some of the more exotic carburetors of the past have taken on legendary attributes. And even though numerous books and magazine articles covering the subject have been written, the myriad past and present carburetor designs and models have generally served to muddy the waters of understanding. The following paragraphs should serve to clear those waters.

Part of the reason that four-barrel carbs are reputed to be complicated, has to do with the fact that the Carter AFB and Holley Model 4150/4160 were the original equipment carburetors on such legendary engines as the 409 Chevrolet, Chrysler Hemi, Z28 Camaro, 427 Ford and Pontiac GTO. Engine legends aside, except for a few eccentric designs, which were originated by equally eccentric engineers, all automotive carburetors produced since the late ‘50s are very similar in concept. Execution varies, but that just means more than one design has been applied to the solution of a single problem.

Back to Basics

The basic purposes of any carburetor, be it a one, two, three or four-barrel, are atomization of fuel, mixing of air and atomized fuel in the amounts required by engine load, and regulation of engine speed. You can understand these concepts most easily by spending a few minutes exploring the theory of carburetion (the mixing of fuel with air) and then relating that theory to functional hardware as incorporated in various carburetor models. As you might expect, you won’t have to read much further before you bump into a few paragraphs that cover these topics.

In order for gasoline to burn within an internal combustion engine, it must be rendered into a near gaseous state by thoroughly mixing it with some amount of air. Air is necessary to provide oxygen, a requisite of combustion. During idle and part throttle operation (such as steady-state cruising and light acceleration) 14.7:1 is the chemically ideal or stoichiometric air/fuel ratio. Under full throttle and heavy load operation, a richer mixture (in the vicinity of 12:1 or 13:1) is required to produce maximum horsepower and keep combustion chamber temperatures (and spark knock) under control. (These air/fuel ratios pertain to straight gasoline—gas/ethanol blends, as in pump gas, which may contain up to 10-percent ethanol, call for a somewhat richer mixture, as does E85 or straight alcohol.)

Suction, caused by the pistons moving down their bores during the intake cycle, creates a partial vacuum in the induction system.  In turn, these low-pressure pulses are communicated through the intake manifold to the carburetor discharge nozzle and cause fuel to flow into the air stream. At this point, the fuel, pouring into the fast moving air, is atomized (converted to a spray of fine droplets), although it has not necessarily been vaporized.  It remains in small droplets until it reaches the low-pressure (high-vacuum) area, beneath the carburetor throttle plates, that enables it to “boil” and become a gas.  Strong manifold vacuum is a necessity, not only for good atomization, but also to provide a proper (low pressure) environment for vaporization, essential for both peak efficiency and maximum performance.

Pressure or Vacuum?

This concept may be more readily understood if compared to an automobile cooling system. Under normal circumstances, water boils at 212 F (100 C), but automotive coolant temperatures frequently exceed that level without the contents of the radiator turning to steam. Were it not for the pressure cap, this would not be the case. (For the purpose of this discussion, assume that the entire cooling system is filled with water rather than a water/antifreeze mixture.)  Pressure keeps the water in a liquid state at temperatures where it would normally turn to steam.

But if even a small amount of pressure is relieved (by lifting a radiator cap’s pressure vent lever) when water temperature exceeds 212 F, steam will hiss out the overflow tube. Similarly, when gasoline flows from above the throttle plate (where pressure is close to atmospheric, i.e., 14.7 psi) to the lower pressure environment of the intake manifold, the gasoline “boils,” changing from a liquid to a vapor.

Mr. Venturi and His Pressure Dropper

Over 150 years ago, G.B. Venturi, an Italian scientist who had no interest whatsoever in carburetors or automobiles, discovered that when air moves through an hourglass-shaped tube (diameter changes from large to small and back to large), the velocity is highest and pressure lowest in the area of smallest diameter. This principle, named for its discoverer, forms the basis for virtually all carburetor design. The low-pressure condition found in the venturi draws fuel out of a reservoir (float bowl) through the discharge nozzle, which is located in the narrow (low-pressure) region of the venturi.

With low pressure in the venturi acting in the same manner as a suction pump, there is no reason for fuel to exit the discharge nozzle in anything other than a liquid state.  Just stick a straw into a glass of vintage wine (rather gauche, but it serves the purpose) and suck on the other end. The principle is identical. Suction is merely a pressure reduction that serves to initiate flow; it provides no means of converting a liquid (be it wine, gasoline or anything in between) to a spray mist. Atomization results when a stream of liquid is introduced into a column of fast moving air, but without an additional step, many of the gasoline droplets would be too large to be vaporized and would remain liquid (and for the most part, unburned) in the combustion chamber.  The additional operation needed in the liquid-to-mist breakdown process is accomplished between the fuel bowl and discharge nozzle, and is known as emulsification or pre-atomization.

When Air Meets Gas

According to the dictionary definition, an emulsion is “a mixture of mutually insoluble liquids in which one is dispersed in droplets throughout the other.” If you were to take quantities of vinegar and vegetable oil, pour them in a bottle and shake vigorously, the end product would be an emulsion (one that we commonly call salad dressing). If the emulsion is allowed to stand, the two liquids will soon separate and the lighter substance will rest entirely on top of the heavier one.

In the world of carburetion, an emulsion refers specifically to droplets of fuel interspersed within a volume of air. The device that is most instrumental in fuel emulsification is known as an air bleed. The air bleed is actually an air metering jet in the end of a passageway connected to the internal channel that brings fuel from the float bowl to the discharge nozzle. The same (low pressure) suction force that causes fuel to flow to the discharge nozzle, also draws air through the air bleed into the liquid fuel stream. The effect is similar to the experience of drawing liquid through a straw that has a small hole in its side. The liquid moves up through the straw, but it reaches your mouth in a bubbly froth. So in a sense, when you drink the wine used in the previously mentioned suction experiment with a straw that has such a hole, you’re drinking emulsified Chenin Blanc, and it is no different from the air/fuel mixture that exits at the main discharge nozzle (with the exceptions of taste and alcohol content, of course).

Let it Bleed

However, the function of an air bleed is not solely to emulsify fuel. It also exerts control over fuel flow by bleeding off some of the suction force or fuel metering signal that is developed at the discharge nozzle. As bleed size varies, so does the amount of suction (vacuum) required to start fuel flow. All other specifications being equal, a larger diameter air bleed increases the amount of vacuum required to initiate fuel flow. Conversely, a reduction in bleed size reduces the vacuum signal requirement for initiating fuel flow.

The function of an air bleed is commonly misunderstood, and that has led a large number of carburetor modifiers to become proficient in the creation of scrap metal. Before picking up a drill bit and attacking an air bleed consider: If the bleed could be infinitely enlarged, the point would be reached where suction pulse was never transmitted to the fuel in the float bowl. In essence, the entire signal would be bled off and only air, entering through the air bleed, would flow through the circuit. Total elimination of the bleed, on the other hand, would allow transmission of the full signal to the fuel reservoir.  While this might seem like an excellent means by which to instigate fuel discharge at low manifold vacuum, lack of an air bleed would eliminate the emulsification process and lead to percolation problems.

After an engine has run for some time, the carburetor body becomes extremely warm, and on occasion, downright hot. When the engine is switched off, carburetor heat can temporarily increase since fuel flow stops (the exiting fuel carries off heat with it). The fuel remaining in the float chamber begins to absorb heat from the surrounding carburetor body and in some cases, the fuel will actually boil or percolate, much like water in a coffee pot. Without an air bleed or anti-percolator vent, hot fuel remaining in the main wells could be forced out of the discharge nozzle by the vapor pressures that result from the percolating action. The presence of a vent allows pressure in the main well to escape, thereby preventing fuel from being forced out of the discharge nozzle.

The air bleeds contained in production carburetors are precisely measured restrictions, sized such that fuel flow from the discharge nozzle into the air stream is started at specific vacuum levels. (Note that this discussion pertains to venturi, not manifold vacuum. Venturi vacuum increases with airflow and is therefore greater at wide open throttle than at idle or part throttle. This, of course, is in direct contrast to manifold vacuum, which decreases as the throttle is pushed toward a wide open position).  Without the sophisticated equipment required to perform fuel flow analysis, it is impossible to accurately assess the results of bleed size alterations. Some answers can be found through trial and error testing, with “error” being the operative term. In addition to altering the airflow levels at which fuel begins flowing through the main discharge nozzles, bleed size also affects air/fuel ratios. After a bleed orifice is enlarged, it admits more air than it did prior to being super-sized, and consequently a leaner mixture will result unless the fuel jets are changed.

Booster Venturis–For a Stronger Signal

At this juncture, it should be clear to anyone who has looked down the throat of a modern carburetor that something more sophisticated than a simple “dump tube” mounted in a low-pressure area is required for satisfactory performance on a modern automotive engine. With a mere tube protruding into the venturi serving as the discharge nozzle, inordinately high airflow is required to develop a sufficient metering signal to initiate fuel flow. The restriction created by a standard venturi will simply not provide a large enough pressure drop (in the immediate vicinity of the discharge nozzle) to cause fuel flow at low air velocities. In essence, the pressure drop (and resultant fuel metering signal) needs to be boosted, and the device utilized to perform this function is appropriately called a boost or booster venturi, and it acts as a venturi within a venturi.  There really is a method to all this carburetion madness.

The use of a booster venturi intensifies the fuel metering “signal” in a carburetor, which enhances response to changing operating conditions. Another advantage of the booster concept is that vacuum can be intensified in the area immediately adjacent to the discharge nozzle. By varying booster shape, it is possible to increase signal strength while minimizing loss of airflow capacity. In some carburetors, a double booster is utilized as a means of further intensifying the fuel-metering signal.

However, any booster design is limited in its ability to distribute fuel evenly around the throttle bore. If poor manifold designs compound this fuel distribution problem, carburetor engineers frequently add a tab or mill off a portion of the booster’s trailing edge. These modifications shape the low-pressure area such that exiting fuel is pulled into a more desirable distribution pattern.

Note that some carburetors, particularly the Carter/Edelbrock AVS and Carter Thermo-Quad carburetors, do not have a venturi of any type in the secondary barrels. Fuel is discharged through a simple dump tube that protrudes into the air stream. In spite of appearances, this is not contradictory to the previous explanation of venturi function.  The concept works because on these carburetors, the throttle plates are operated mechanically and the area surrounding the discharge tubes is subjected to manifold vacuum, because it is “sealed” from atmospheric pressure by an air valve held closed by an adjustable spring. Therefore a fuel delivery signal is presented to the discharge nozzles before air begins to flow in any appreciable volume. In this instance, manifold vacuum, not venturi vacuum, causes fuel to flow out of the nozzle. Were it not for the air valve being located above the discharge nozzle, fuel would not flow at low speed and the engine would suffer from an extreme lean condition as the secondary throttle is opened.



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