Gasoline needs to be broken down into small particles (atomized), and mixed with air (emulsified) to burn. This is the task of the carburetor.
Thermodynamic law says that energy cannot be consumed or destroyed, only its state can change. An engine adheres to this theory by turning heat into motion, and back in to heat again.
Beyond supplying the fuel charge, the carb also throttles the engine, controlling its operation at different speeds. Throughout the operating ranges, requirements vary in regard to the amount of fuel mixed with air. Identified as the air/fuel ratio, it is referenced in parts of air to a constant part of fuel. The term "stoichometric" identifies the ratio offering the most complete burn. For unleaded gasoline, it is 14.67:1, which is commonly rounded to 14.7:1. The stochiometric value for other fuels varies with their energy content, but our discussion will be limited to the requirements of gasoline.
For an engine to produce peak power, it needs an air/fuel ratio richer than stoichometric, usually 12.7 to 12.9:1 at peak torque and 13.0 to 13.2:1 at maximum horsepower. This may seem to be a contradiction to the definition of stoichometric, but it's a byproduct of the inherent faults of piston engine design. Many think increasing fuel throughout the operating range of an engine will make it more powerful. This is false, as the most power will be developed when there is a sufficient amount of fuel present to burn all of the oxygen in the bore, not the other way around. The amount of fuel fed to the cylinders can be altered by jet sizing, but the volume of the air pumped is fixed. The statement "lean is mean" recognizes this, but is one-dimensional. It ignores the fact that when the cylinder is filled, the bore is never completely empty of exhaust byproducts that dilute the fresh charge, and that some of the mixture falls where the flame front never travels. In an ideal model, a gas engine would want a 14.7:1 ratio of for peak power, but this is reality. When the mixture ratio wanders from optimum, there will be either excess oxygen or excess fuel in the bore. This can be expanded to define the combustion event as having either not enough fuel to burn all of the oxygen (lean), or an insufficient supply of air to burn all of the fuel (rich). Either way, power drops off.
Additional circuits of the carb alter mixture ratios and are categorized as the accelerator pump and power enrichment. The accelerator pump is used to fill the void created in fuel delivery when the throttle plates are opened rapidly. Enrichment by a power valve or metering rods references engine load through vacuum, and offers additional fuel beyond the main metering circuit. It is evoked when the throttle angle is steady, but the load on the engine increases. Idle is the richest state in a carburetor; the throttle plates are closed and the engine is experiencing the lowest possible volumetric efficiency. As the throttle opens, the mixture can be leaned. To produce maximum power, it needs to go rich.
As with any mechanical apparatus, the carburetor takes on various forms and designs to accomplish the same task. Even with these application-specific changes, the core functions remain constant. Servicing and tuning a carb pays huge dividends in idle quality, throttle response, fuel economy, and overall performance. Cleanliness is important, and the carburetor should be kept free of varnish deposits with a good spray cleaner, with special attention paid to the air bleeds.
THE FLOAT SYSTEM
The fuel level in the float bowl is important to the operation of the carburetor; it dictates the throttle angle required to pull fuel into the main metering system. Lower-than-specification float levels will delay pullover, creating a possible lean condition on throttle tip-in that could be misinterpreted as a weak accelerator pump signal. Conversely, a high float level will cause early pullover and a possible rich tip-in condition. The Demon and Holley-style carbs feature external float adjustment through either a sight plug (Holley), or sight glass (Demon). Multipiece designs (like the Edelbrock or Rochester Q-Jet) require the air horn to be removed and the float level adjusted by bending a tang while being checked with a ruler. If the float is attached to the air horn (Carter style), then there is usually a height and drop adjustment.
PHR Tuning Tip: Always adjust the float level before making any other changes. Make sure the bowl vents are clear and operating properly.
THE CHOKE SYSTEM
Most aftermarket carburetors do not have a full choke plate. Instead, a machined groove or passage allows air to pass when the butterfly is closed. These styles usually offer no adjustment beyond the thermostatic (heat-activated) spring tension and fast idle. Spring tension is adjusted by rotating the spring housing. This alters the spring tension, changing the time required for the spring to heat up, uncoil, and open the choke. Factory-style carburetors usually have similar settings, but also feature a full choke plate that closes off all air when shut. This requires a vacuum-operated choke pull-off. As soon as the engine starts, the fast idle speed causes the choke plate to blow open a small amount, and it will be further opened with the vacuum break. The effect the pull-off has over the opening is usually adjustable, and is responsible for a clean fast idle with no loading.
PHR Tuning Tip: Fast idle speed is critical to both styles of choke plate. When a pull-off is used, set the fast idle to specifications before making any adjustments.
THE ALL-IMPORTANT IDLE MIXTURE
The idle screws are needle valves that limit the amount of fuel passed through the idle orifice. Except for some older reverse-idle-circuit Holleys, when the screws are turned in, the mixture will be leaner. Many designs use only two idle screws located on the primary bores, while all Demon carbs and some Holleys offer four-corner idle mixture adjustments, creating better fuel distribution, and a more accurate setting. Before adjusting the mixture, the idle speed needs to be set. This will set the proper amount of throttle opening so the booster signal is accurate. Ignition timing will impact idle speed and should be set prior to any adjustments. Seat and turn each idle mixture screw out the same amount; two turns off the seat is usually a good starting point. With a tune-up-style tach connected, slowly lean the mixture until the highest rpm is recorded, turning the needle valve in 1/8- to 1/16-turn increments. This will produce the best idle and leanest possible air/fuel ratio. If idle speed becomes excessive, readjust the throttle plates before continuing.
PHR Tuning Tip: It is best to do this with the air cleaner assembly in place, and if the car is equipped with an automatic transmission, it should be in drive with the wheels chocked. This will duplicate the actual pressure drop across the carb.
THE MAIN METERING SYSTEM
Changes to jets alter the air/fuel ratio throughout the rpm range. This is usually best performed at the track or chassis dyno with an air/fuel meter attached to the exhaust. Jet size is affected by the engines ability to convert fuel to power, and is qualified under the term brake specific fuel consumption (BSFC). Measured in pounds of fuel to produce 1 hp, BSFC can vary from .35 to .60 pounds for each horsepower. A good baseline is .5 pound of fuel for each horsepower. Compression ratio and cylinder head materials have a big impact on BSFC. As compression goes up, the BSFC goes down, representing a more efficient burn. Aluminum dissipates more heat than cast iron, increasing the BSFC number.
PHR Tuning Tip: Always tune from rich to lean, listening for detonation and tuning the spark advance curve along with the carburetor jet size.
FUEL ENRICHMENT CIRCUITS
Power valves are used by both Demon and Holley, and feature backfire protection to avoid premature failure. They are rated by the amount of manifold vacuum signal required before being opened and enriching the mixture. As an example, if marked 6.5, the additional enrichment would not be added until engine vacuum degraded to 6.5 inches of vacuum. The higher the number, the earlier the power enrichment will happen. Edelbrock, Carter, and Q-Jet carbs use metering rods that are attached to a power piston, restricting the main jets. These styles are not as easily tuned to vacuum, but they offer a multitude of changes to the air/fuel ratio by altering metering rod diameter. Keep in mind that as the metering rod is made thinner, the mixture goes richer, since more jet area is exposed.
PHR Tuning Tip: Enrichment circuit tuning is essential for good transition from primary to secondary circuits. Pay particular attention to power valve ratings for engines with large cams and little vacuum, and tow vehicles that need enrichment under loads.
TUNING THE ACCELERATOR PUMP
The old REO-style (Ransom E. Olds, the founder of Oldsmobile and REO Trucks) accelerator pump is very tunable and is offered by both Demon and Holley. This allows for changes in the pump volume, the rate of travel with throttle movement, and the squirter design and position. OE-based carbs usually incorporated plunger-style accelerator pumps and had relatively limited travel in relation to throttle position.
PHR Tuning Tip: With the float bowl full and engine off, check for a powerful steady stream from the accelerator pump squirter. Engines with poor booster signals and low vacuum usually require excessive accelerator pump stroke to afford good throttle response and driveability. Tune this circuit with a vacuum gauge attached to the engine so that a good relationship between the accelerator pump and power valve opening can occur.
THE SECONDARY METERING SYSTEM
Tuning consists of jet size, metering rod diameter (if equipped), rate of gain, and timing of the opening. Most aftermarket carbs feature a vacuum-operated secondary, utilizing a diaphragm. Q-Jets incorporate a mechanical link to the secondary throttle plates, controlling additional fuel by allowing an air door to sense the requirements of the engine and lifting the metering rods from the secondary jets. This design has pressed-in jet orifices, but the externally located metering rods are easily changed. Spring tension on the air valve can also be altered. The rate of gain for the metering rod lift is adjusted through secondary cam changes. Modular carburetors are usually less complicated to tune, offering either a mechanical secondary that allows only jet changes, or vacuum-operated secondary that also include alterations to the opening rate through changes to diaphragm spring tension.
PHR Tuning Tip: Jets change the amount of fuel, but do not forget to tune with air bleeds and emulsion orifices to mix the fuel and air at different rates.
Here at PHR, we realize we could never supply all the information needed to understand and tune each carburetor in a single story. If this has whetted your appetite for more knowledge, we suggest that you invest in one of the many books on the market that are dedicated to a specific carburetor's tuning and modifications. For those of you with Demon carburetors, a complete tuning guide became available in fall 2000 from SA Design Books.
There's a lot going on inside a carburetor, as shown with this cutaway of a Demon.
This Demon is having its air bleeds cleaned with a spray carb cleaner, an essential servic
Carburetors that suspend the float from the air horn usually require a float height and dr
To adjust the drop, the tang by the pivot holes is tweaked, while the height is manipulate
Modular carburetors use sight windows and require the float level to be set with the engin
Most chokes are adjustable for spring tension. The Holley has two reference marks, one on
All Demons and some Holleys offer four-corner idle circuits. When adjusting the mixture on
Here, some jet extensions are being installed in a Demon to eliminate fuel starvation on l
Modular carburetors employ a power valve to provide enrichment for engine load and for a s
The REO style external accelerator pump is used on modular carburetors such as the Demon a
Rochester, Carter, and Edelbrock use a plunger style accelerator pump. Shown is the Edelbr
Carter, Rochester, and Edelbrock use metering rods to provide enrichment for both primary
The primary side is triggered from vacuum, while the secondary is evoked in by air flow pa
A unique feature of Demon RS models is the tuneability of the booster signal with removabl
When tuning a modular carburetor, invest in non-stick gaskets to make for a quicker and cl