connected to a common rod, forming four pressure chambers when assembled. The outer end of the rod connects to the fuel metering servo valve that moves away from the throttle body to open, allowing more fuel flow or toward the throttle body to close, reducing the amount of fuel to flow. The rod is moved by the forces measured within the four pressure chambers. The smaller components of the carburetor are either attached to, are a part of the major portions, or are remotely mounted, depending on the engine application. The boost component is mounted on the inlet side of the throttle body. It measures air density, barometric pressure, and air flow into the carburetor. It is mounted directly in the air flow at the inlet to the throat. The automatic mixture control, if equipped, is mounted either on the boost portion for throttle bodies with two or more throats, or on the throttle body itself for the single throat models. The fuel delivery component is either remotely mounted at the "eye" of the engine's supercharger or at the base of the carburetor body. The fuel is sprayed into the air stream as it enters the engine through one or more spring-controlled spray valves. The spray valves open or close as the fuel flow changes, holding a constant fuel delivery pressure. An accelerator pump is either remotely mounted or mounted on the carburetor body. The accelerator pump is either mechanically connected to the throttle, or it is operated by sensing the manifold pressure change when the throttle is opened. Either way, it injects a measured amount of extra fuel into the air stream to allow smooth engine acceleration. Military carburetors may have an anti-detonation injection (ADI) system. This consists of a "derichment valve" in the fuel control component, a storage tank for the ADI fluid, a pump, a regulator that provides a specific amount of ADI fluid based on the fuel flow, and a spray nozzle that is mounted in the air stream entering the supercharger. Theory of operation There are four chambers in the fuel regulator portion of the carburetor. They are referred to by letters A, B, C, and D, with the A chamber closest to the throttle body. The fuel metering servo valve responds to pressure differentials across the diaphragms separating the chambers. The resulting diaphragm movement controls fuel flow into the engine under all flight conditions.