Here, you can see the gauge in an SMI carb. The venturi was opened up enough to allow the smaller portion of the gauge through, but the couple extra thousandths of an inch of thickness was enough to keep the big end from going through.
 These are the idle (left, notice the larger diameter) and high speed (right) air bleeds. These are important tools in tuning the fuel curve for different applications. The high speed is very influential on the curve at the top one-third to one-quarter of the rpm range. A change of a few thousandths of an inch of the diameter of the bleed can make a very large change in the air/fuel ratio.
 This is a shot of SMI's APS (anti pull-over system) kit. The custom kit replaces the regular weighted pin and accelerator pump discharge nozzle screw with a spring-loaded check ball. The idea is that the check ball helps maintain positive pressure to stop unwanted fuel discharge from the nozzle at high rpm levels. This happens to a lot of V-8 applications and almost every time on four-cylinder racing engines. The reason for this undesirable situation occurring on V-8 engines is because the extremely high air speed—far greater than what the carburetor was ever designed for—literally turns the discharge nozzle into a "booster." The pressure drop acting on it is great enough to pick the factory weighted pin up off its seat and draw fuel through the pump circuit. In four-cylinder applications the engine's harmonics (vibrations) "dances" the pin off the seat and the fuel right out of the nozzle—even at idle. Oddly, Murphy says that, in some cases, the condition calms down at higher rpm levels because the time between firing pulses is reduced and the engine becomes more "balanced."
 These are the three types of accelerator pump discharge nozzles. Starting from the left, that's an anti pull-over discharge nozzle, a standard, and a tube-type. Never use a tube-type as they increase the chance of pull-over. That happens because the tubes are placed further out of the air stream. The anti pull-over type of discharge nozzle is rarely effective and only applies to the extreme airflow situation created by the V-8 engine anyway.
 This photo shows the two power valve channel restrictors (PVCR), which are the two small holes on either side of the large hole in the center for the power valve. They work in conjunction with the main jets to provide fuel delivery to the main circuit. The power valve is a normally open valve that is controlled by the manifold pressure (vacuum). At idle and light throttle—such as when you are running around under caution—it is held closed because the throttle is partially closed creating vacuum in the plenum of the intake manifold. At full throttle the valve opens via spring pressure since the fully open throttle removes the restriction to airflow which gets rid of the vacuum condition in the intake. Except in V-8 applications running a two-barrel the carburetor is undersized and acts as a restrictor to the intake tract air flow. Therefore, at high rpm levels positive manifold pressure (vacuum) is created because the cylinders pull more air than the carburetor can provide, even at wide-open throttle. Think of it as trying to drink a milkshake through a cocktail straw. Your mouth is the manifold plenum and the straw is the carburetor. In many cases the vacuum created will be strong enough to overcome the power valve's spring and the valve will close. A good tuner will use this to their advantage. At high rpm the pressure drop (or signal) at the booster is far beyond anything that the carburetor was designed to handle thanks to the extreme volume and speed of the air travelling through the venturi. The result is excessive fuel delivery from the booster. To help control this situation a tuner will use the spring pressure of the power valve to determine the "timing" when the valve closes at high rpm to manipulate the fuel curve to keep the carburetor from going rich at high rpm levels. To get this right the PCVR size must also be considered, but in two ways—how much fuel it provides when it opens at low rpm, as well as how much fuel it takes away when it closes at high rpm.
 These are the emulsion holes. Think of them as holes in a garden hose, and the main circuit is the garden hose itself. The emulsion system works in conjunction with the high-speed air bleed to help mix oxygen and emulsify the fuel prior to being discharged from the carburetor. The emulsion holes also have an effect on when the main circuit activates and affects how pressure drop at the booster results in fuel delivery. As mentioned before, the emulsion holes have the same effect on the carb as you trying to such water through a hose that has holes in it. The larger the diameter of holes and the more holes you have, the harder it is to suck water through the hose. This can be helpful when tuning a carburetor that is undersized and sees a very high pressure drop across the venturi booster because of it.
 Although it's listed as a 500-cfm carb, the Holley 4412 is almost completely identical to the primary side of the Holley 4779, which is a 750-cfm mechanical secondary four-barrel carb—aka the "750 Double Pumper." This is because two- and four-barrel carburetors are tested differently.