by Barry Grant Inc. ©2004
Given that a four-barrel carburetor possesses something in the order of 200 components to allow it to function correctly, it’s no wonder the average enthusiast has the odd question about how it operates and why. Words and phrases like atomization, vacuum signal, fuel-metering circuits, etc. can be a little bewildering. So, here are a few of the basics with several useful illustrations to help explain. This article contains information that applies to a wide range of V8-engined machines, from street cars to tow vehicles, and from oval track racers to drag strip cars.
1) Q. Is air pushed or sucked into a carburetor?
A. With the exception of forced induction systems (centrifugal supercharger, or turbocharger – where air is forced through the carburetor), air is drawn through the carburetor (sucked) by the engine’s vacuum signal. The amount of air is determined by the strength of the vacuum signal.
2) Q. What is the difference between the straight-leg venturi booster, the down-leg booster, and the annular-discharge booster? Also, when should one be used in preference to another, and how does it affect jetting?
A. A straight-leg booster has, as its name implies, a straight leg, which protrudes from the body of the carburetor into the main venturi. Its discharge ring is situated slightly above the venturi’s most effective zone. Its discharge ring is slightly above the venturi’s most effective zone. The color red indicates the main fuel circuits, which connect with the main jets, the emulsion holes (blue) and the high-speed air bleeds located in the air entry on top of carburetor. The color yellow indicates the idle-fuel circuits, which connect with main-fuel wells, the idle-feed restrictors, the idle-air bleeds (also located in air entry on top of carb), and the idle-discharge ports and transfer slots in the baseplate.
B. A down-leg or drop-leg venturi booster drops the discharge ring lower in the carburetor’s main venturi where it operates in air of higher velocity, which draws more fuel than the straight-leg style of booster.
C. The annular-discharge venturi booster has a larger ring with multiple discharge holes rather than the single outlet hole of the straight-leg or down-leg style of booster. This has the effect of creating a venturi within the main venturi that produces greater vacuum than either the straight- or down-leg varieties and draws even more fuel.
D. In conclusion, the pros and cons of each type of booster are based largely upon its application. However, if each style is tested in the same carburetor with all else being equal, the down-leg booster will require smaller jets to flow the same amount of fuel than a straight-leg booster. And the annular-discharge booster will require even smaller jets to flow the same amount of fuel as the down-leg type.
3) Q. Mechanical secondary or vacuum secondary, which style of carburetor should I use and why?
A. Generally speaking, a mechanical secondary carburetor is preferred on vehicles with manual transmissions and on automatic transmissions with 3000+ RPM stall-speed converters. For automatics with less than 3000-RPM stall-speed converters, the vacuum-secondary carburetor is usually the better choice.
4) Q. How do I set the float levels and what effect do they have?
A. On Demon carburetors in street-driven applications, begin by setting the float levels at _ distance of the sight window. On race applications, increase the levels by setting them between _ and _ distance in the sight window. On Holley carburetors, remove the level plug and adjust the float level until fuel trickles from the open port. Changing the float levels alters the amount of fuel in the bowl (reservoir) and the carburetor’s ability to feed the main jets. By raising the float level the engine’s response is quickened. This is an adjustment frequently used to eradicate a lean stumble. By lowering the float levels, the activation of the main metering circuit is delayed and consequently produces a leaner mixture coming off idle. For street applications the latter condition is more economical.
5) Q. Does a bigger carb make more power? What’s the limit?
A. A larger carburetor can produce more power on a dynamometer, but under normal operating conditions can result in slower acceleration and lower efficiency of fuel atomization. Select the smaller carburetor, especially if you’re undecided about sizes. The smaller diameter venturii increase the velocity of the air/fuel mixture. As a consequence, it usually provides better acceleration and proves to be more efficient.
6) Q. Where should the fuel-pressure regulator be located and what type should I use?
A. The pressure regulator should be mounted close to the object (carburetor, nitrous system, etc.) that’s being fed with fuel. The further away the regulator is placed the greater the delay in its response (its opening and closing). A slower response causes fluctuations (spikes) in the fuel pressure. Regarding the selection of a regulator, specific vehicle requirements, but especially the type of fuel pump already in use dictates the type of fuel-pressure regulator required. For example, combine a block-mounted 15-psi pump with a throttle bypass; a belt-driven pump with a diaphragm bypass; a BG280 electric pump with a two-port regulator and a BG400 with a four-port regulator.
7) Q. How does weather and altitude affect carb jetting?
A. The more oxygen there is in the air the more fuel the engine will demand. For example, as the weather becomes colder (winter) or the altitude lower (closer to sea level), the air will contain more oxygen and the engine will require a larger jet size. In contrast, as the weather becomes warmer (summer) or the altitude higher (mountainous) the jet size needs to be reduced.