During the late '90s, carburetor tuning and modifying received something of a jolt when Barry Grant introduced his racing four-barrel, the Demon. The innovations included streamlined air intakes, concentric venturis, removable-booster venturis, billet-metering blocks, float bowls with sight glasses, and a billet baseplate. It's uniqueness was further exploited when the RS version (removable venturi sleeves) burst upon the racing scene. Furthermore, in an attempt to prevent ridges in the venturi bores and to obviate the potential of core shift during the casting process, the main body was formed using a method known as Concentracast.

The advantage of having smooth concentric carburetor bores was obvious, but there was a secondary prize: consistency of measurement. The selection and tuning process of the new Demon, especially the RS model, was also far-reaching. Whether prevailing conditions or, perhaps, engine size dictated a change, having the option of replaceable venturi sleeves meant that the carb could be instantly resized without being replaced.

Carburetor Selection
Small carburetors tend to provide better throttle response at lower engine speeds, and bigger carburetors are inclined to produce more power in the mid- and upper-rpm ranges. When running on a dry, slick racetrack, a large carburetor that produces less power at lower rpm may be the ideal solution for keeping the car "hooked up" on corner exits. Conversely, on a tacky racetrack, a smaller carburetor delivering lots of bottom-end power will provide strong acceleration from the corners. During the spring and fall, or when the track is sticky, some racers will qualify with a smaller carburetor, then switch to a larger model during the evening if the track becomes slick. Where restrictions apply, and the rule book only permits the use of one specific carburetor, it's advisable to have it blueprinted and flowed to obtain maximum performance.





A common misconception about carburetors is that the greater signal (vacuum) and fuel draw created by venturi boosters, the more power the engine will produce. Furthermore, since the annular-type boosters draw more fuel with less air speed than the straight-type or the drop-leg booster, why not install them in all racing carburetors? This reasoning is entirely understandable. However, although annular-style boosters function well on some applications, they can be restrictive and/or create a disproportionate fuel curve (frequently too rich at high revs) on others. In order to have a carburetor perform to its full potential, the combination of airflow and fuel metering must be of the correct proportions throughout the entire rev range.

A venturi booster with excessive draw, as indicated above, will cause the fuel curve to climb and become overly rich as engine speed increases. The result is a loss of torque and horsepower on the straightaway. Reducing the jet size, in an attempt to correct the mixture, will cause a loss of power on corner exit. The best choice of carburetor is one with scientifically shaped venturis that can flow sufficient quantities of air at higher revs, without sacrificing acceleration in the lower rpm band.

Additionally, the booster configuration and the calibration of the metering circuits must provide proportional fuel draw throughout the entire rev range of the engine. A good technician at any quality carburetion and fuel-system shop can advise as to which carburetor and venturi booster combination is best suited for a particular engine size, track length, and track condition.

Idle Adjustments
Instant part-throttle response is inherent in all successful oval track cars. Idle-mixture adjustment plays an important role in achieving it. Many racers seem to feel that if they take their foot off the accelerator pedal and the engine doesn't quit, the car is idling fine. The truth is that initial acceleration, throttle response, and smooth deceleration are all related to a properly balanced idle system. On most carburetors, the fuel controlled by the idle-mixture screws is discharged from one or more holes in the baseplate below the butterflies. This provides sufficient fuel to run the engine when the butterflies are virtually closed. As the throttle opens, the butterflies uncover the transfer slots, which provide fuel for the initial surge of air entering the manifold.

Carburetors not responding to adjustments of the idle-mixture screws usually have too much of their transfer slots exposed at idle. If this situation exists, the transfer slots are probably already open and no fuel is available to mix with the incoming air until the discharge from the accelerator pump arrives. This condition is chiefly responsible for off-idle stumbles and hesitation. With the idle circuit correctly calibrated, control over the curb-idle circuits will be restored, the engine will run with the transfer slots closed, and a smooth transition from closed to fully-open throttle is assured.

During deceleration, the carburetor will be exposed to extremely high manifold vacuum. If the butterflies cannot close fully during deceleration, excessive fuel can be drawn from the transfer slots and cause flooding and backfiring. Flames appearing from the exhaust during deceleration are usually a good indicator that the idle circuit is not properly calibrated and the butterflies are not restricting the supply of fuel from the transfer slots. Obtaining a good idle also depends upon the correct adjustment of the float levels.

Float Level
Incorrect float levels allow the carburetor to either flood or run out of fuel. Float levels are subject to two settings: the initial factory setting, and a final adjustment once the carburetor is mounted on the manifold with the engine running. The initial procedure is conducted by removing the float bowl from the carburetor main body, turning it upside-down, and measuring the gap between the float and the bowl. The gap should be 0.4 inch and can be measured by either a ruler or a caliper. Final float level adjustments of a Demon are simply made by setting the fuel level to the top line of the viewing window. To check float levels on carburetors without sight glasses, remove the float-level sight plug while the engine is idling. Take care that fuel doesn't spill and create a fire hazard on a hot engine. Fuel should barely trickle from the primary or front end of the carburetor and should be slightly higher at the rear. When turned clockwise, the hexagon nut on the needle-and-seat will adjust the float downward, and upward when turned counterclockwise. To adjust the needle-and-seat, slightly loosen the screw in the middle of the assembly and retighten it when adjustments have been completed.

Power-Valve Adjustment The power valve provides a way of leaning the fuel mixture to the engine under low- and no-load conditions. Power valves are rated in inches of vacuum and numbered accordingly. When the engine manifold vacuum drops beneath the number stated, the power valve opens and enriches the main circuit. To keep the idle clean and sharp, the power valve should remain closed during idling. To check the power valves, use a vacuum gauge and read the engine's vacuum at idle. The power valve number should be 1.5 to 2 inches of vacuum under the engine's manifold vacuum. For example, if an engine idles at 8 inches of manifold vacuum, a 6.5 power valve would be appropriate. The power valve will not open and enrich the circuit until the engine vacuum drops to 6.5, as it decreases toward zero vacuum when the throttle is opened to or near full throttle. This ensures the power valve will remain closed at idle, which keeps the engine clean and the spark plugs from fouling.

Getting the right mixture
Maximum horsepower is essential for winning races, and the correct fuel mixture is essential for making maximum horsepower. Proper jetting is one of the major tools used to accomplish this.

Fuel, oxygen, and a heat source are the three components necessary for combustion. The heat source (the ignition system) ignites the combustible charge, and the fuel and oxygen burn to create energy. The amount of energy that can be produced is based on the amount of fuel that is burned, which depends upon the oxygen available. The density or weight of the air affects the amount of oxygen carried in the air. The three factors that influence air density are temperature, barometric pressure, and humidity. Higher temperature, lower barometric pressure, and higher humidity all cause a decline in air density. Therefore, air density increases with lower temperature, higher barometric pressure, and lower humidity. As air density increases, more fuel can be burned and power output increases. As air density decreases, less fuel can be burned and power output drops.

This is why lap times often show substantial improvements when the heat of the day fades and the coolness of the evening sets in. Racers and engine tuners can monitor these changes in air density and use the information to calculate fuel-mixture adjustments. Chassis or driveline changes may also be necessary to compensate for higher or lower power levels.

SOURCE
Barry Grant/Demon Carburetion
Dahlonega
GA
7-06/-864-8544
barrygrant.com
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