The ideal positioning at idle:...
The ideal positioning at idle: butterflies almost fully closed, transfer slots slightly exposed (shown below the pointer), and an idle-discharge port poised to present idling fuel. This image also shows a Race Demon billet baseplate viewed from under the butterflies.
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.
Plenty can be accomplished...
Plenty can be accomplished with the versatility of a modern carburetor. Removable venturi sleeves encourage resizing and re-tuning for changing weather or track conditions or even a change of engine.
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.
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.
Float level adjustment is...
Float level adjustment is simple, and the effects can greatly enhance the carburetor's performance.
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.