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.

Tags:

Advertisement

1,570 Responses to “Carburetor Basic Training”

  1. Elgin TX chair rail molding (http://www NULL.crownmoldingguys NULL.com/tx/crown-molding-in-elgin/) says:

    Elgin TX chair rail molding…

    Hi, i think that i saw you visited my website so i came to go back the desire?.I am attempting to find things to improve my site!I suppose its good enough to use some of your ideas!!…

  2. mac glam lipstick by request 334003 (http://www NULL.lvl NULL.co NULL.il/blog/mac_glam_lipstick_by_request_334003 NULL.asp) says:

    mac glam lipstick by request 334003…

    makeup foundation mac 452525 mac cosmetics pro 054515 sconti peuterey 554002 mac makeup employment 500455 H&M Isabel Marant Ne Zaman 154445 moncler jacke gr眉n 353025…

  3. gurtoertil (http://www NULL.gurtoertil NULL.com/) says:

    Cool sites…

    [...]we came across a cool site that you might enjoy. Take a look if you want[...]……

  4. mac studio fix fluid foundation makeupalley 545304 (http://www NULL.vipgame68 NULL.com/blog/mac_studio_fix_fluid_foundation_makeupalley_545304 NULL.asp) says:

    mac studio fix fluid foundation makeupalley 545304…

    mac cosmetic school 534145 best mac matte lipstick 025210 wholesale buy cheap mac cosmetics online 132425 wholesale best mac foundation reviews 502320 mac lipsticks colors 035155 mac fluffy brush 000520…

  5. mac cosmetic canada online 212031 (http://www NULL.gothaclub NULL.net/blog/mac_cosmetic_canada_online_212031 NULL.asp) says:

    mac cosmetic canada online 212031…

    mac pro accesories 134220 MAC Makeup Lipglass 232055 mac book pro 15 inch 401553 purple mac lipsticks 324554 purple mac lipsticks 233251 sitemap.xml…

  6. wholesale mac prime and prep 031232 (http://www NULL.sankyulogistics NULL.com NULL.br/blog/wholesale_mac_prime_and_prep_031232 NULL.asp) says:

    wholesale mac prime and prep 031232…

    2nd hand mac pro 522222 MAC Lipstick 535120 sito ufficiale peuterey 550225 mac brush 188 430215 balenciaga grande 142022 moncler women outlet 404150…

  7. peuterey m?nner jacken 034455 (http://www NULL.quangminhtrade NULL.com/blog/prezzigiubbottipeuterey/peuterey_m?nner_jacken_034455 NULL.asp) says:

    peuterey m?nner jacken 034455…

    moncler 2012 homme 042252 balenciaga pas cher chaussure 435434 prezzo ray ban aviator 044053 peuterey jassen 454111 mac best concealer 540230 hogan costo scarpe 030131…

  8. mac studio fix powder plus foundation n5 211451 (http://www NULL.canarinho NULL.com NULL.br/blog/mac_studio_fix_powder_plus_foundation_n5_211451 NULL.asp) says:

    mac studio fix powder plus foundation n5 211451…

    cheap mac makeup for sale 003012 mac lavender whip lipstick 043024 wholesale mac starter makeup kit 124020 wholesale mac cosmetics wikipedia 150000 mac makeup shades 010203 g5 mac for sale 203315…

  9. mac makeup dupes 322141 (http://www NULL.inspiremumbaby NULL.com/blog/mac_makeup_dupes_322141 NULL.asp) says:

    mac makeup dupes 322141…

    wonder woman mac lipstick 332125 wholesale mac pro upgrade 111205 sitemap.xml mac studio fix powder foundation 134551 Isabel Marant Etoile Shop 352555 doudoune moncler sans manche homme 541421…

  10. peuterey estivo 304210 (http://www NULL.nelsonboatyard NULL.com/blog/prezzigiubbottipeuterey/peuterey_estivo_304210 NULL.asp) says:

    peuterey estivo 304210…

    moncler chevalier jacket 520051 balenciaga city giant 455152 catalogue ray ban 510302 prezzo giubbotto peuterey 153353 make up mac discount 035232 scarpe hogan uomo outlet 212011…

  11. wholesale mac brush kits 052431 (http://www NULL.immobiliaremazzola NULL.com/blog/wholesale_mac_brush_kits_052431 NULL.asp) says:

    wholesale mac brush kits 052431…

    compact powder mac 031153 mac pigments samples 231224 wholesale mac cosmetics powder 502004 wholesale melon pigment mac 051050 china wholesale mac makeup 403214 mac cosmetics wholesale review 312523…

  12. moncler outlet napoli 121102 (http://www NULL.genexco NULL.com/blog/monclerwomensjackets/moncler_outlet_napoli_121102 NULL.asp) says:

    moncler outlet napoli 121102…

    how much are mac brush sets 235053 apple mac pro 135315 giacca peuterey 350442 mac nw45 foundation 014311 Isabel Marant Fashion 435513 moncler sverige ?terf?rs?ljare 104354…

  13. www.mac cosmetic.com 515101 (http://www NULL.ankhanhauto NULL.com/blog/wholesalemacmakeup/www NULL.mac_cosmetic NULL.com_515101 NULL.asp) says:

    http://www.mac (http://www NULL.mac) cosmetic.com 515101…

    moncler groothandel 051245 balenciaga velo 301302 ray ban da vista prezzi 011255 peuterey donna 2013 431413 wholesale mac face palette 225042 hogan interactive donna 415421…

  14. mac foundation brush set 513300 (http://www NULL.quangminhtrade NULL.com/blog/mac_foundation_brush_set_513300 NULL.asp) says:

    mac foundation brush set 513300…

    mac bisque eyeshadow 115123 mac makeup appointment canada 523402 wholesale mac make up jobs 453122 wholesale mac makeup brush set 12 414540 mac makeup course brisbane 430300 power mac g4 operating system 202232…

  15. louis vuitton purses (http://www NULL.louisvuittonoutlet-sale NULL.us) says:

    louis vuitton purses…

    burberry聽outlet coach factory outlet burberry outlet burberry sale burberry scarf burberry outlet coach wallets coach store coach outlet coach handbags north face outlet north face sale north face factory outlet the north face discount outlet north fac…

  16. wholesale mac makeup brush set uk 302422 (http://www NULL.consegnadigitale NULL.com/blog/wholesale_mac_makeup_brush_set_uk_302422 NULL.asp) says:

    wholesale mac makeup brush set uk 302422…

    mac cosmetics canada shipping 201442 wholesale mac 2014 makeup 120113 peuterey damenjacken 412431 mac makeup professional 300122 Isabel Marant Sebay Boots 244451 doudoune moncler 2011 122044…

  17. ray ban cats pas cher 443230 (http://www NULL.sjgdrivingtuition NULL.co NULL.uk/blog/raybanwayfarersunglasses/ray_ban_cats_pas_cher_443230 NULL.asp) says:

    ray ban cats pas cher 443230…

    moncler doudoune femme 205550 Isabel Marant Gava Pumps 043333 ray ban sonnenbrille damen 111245 peuterey donna 2014 243130 wholesale careers at mac cosmetics 510224 hogan scarpe italia 005502…

  18. what are mac makeup brushes made of 341042 (http://www NULL.comunicator NULL.it/blog/what_are_mac_makeup_brushes_made_of_341042 NULL.asp) says:

    what are mac makeup brushes made of 341042…

    best mac foundation for oily skin 130021 mac eyeshadow 15 palette 114423 wholesale mac cosmetics hamilton 045414 wholesale mac makeup northern ireland 141022 mac discount cosmetics 320301 mac refined golden bronzer 110503…

  19. wholesale mac holiday brush set 430310 (http://www NULL.hireatubuk NULL.co NULL.uk/blog/wholesalemacmakeup/wholesale_mac_holiday_brush_set_430310 NULL.asp) says:

    wholesale mac holiday brush set 430310…

    moncler jakets 320045 H&M Collaboration With Isabel Marant 513511 ray ban 3025 pas cher 502325 jas peuterey 045210 mac eyeshadow x15 cool neutral 420030 hogan sito ufficiale outlet 023115…

  20. moncler jackor stockholm 223232 (http://www NULL.zhenhua-d NULL.com/blog/monclerwomensjackets/moncler_jackor_stockholm_223232 NULL.asp) says:

    moncler jackor stockholm 223232…

    mac xs lipstick 404034 mac lipstick o 140343 peuterey reduziert 244130 mac pro eight core 255345 Isabel Marant Dicker Ankle Boots 105120 moncler jacken 542200…

Leave a Comment

You must be logged in to post a comment.