Early 911 Enthusiast
Early 911 Enthusiast
Weber Triple-Barrel Carburetor Glossary
Accelerating Pump
The accelerator pump is composed of plunger diaphragms, springs and housing. During operation a plunger pushes a diaphragm inside the housing which moves fuel 
through the accelerating pump ducts. The fuel travels through the delivery valves to the pump jets and is squirted into the throttle body. The accelerating pump can be adjusted with a threaded shaft attached to the cam mechanism.
Air Compensation Screw
The air compensation screw needle valve is actuated by turning it in to close and out to open. It is normally closed and is opened to adjust airflow relative to other throttle bodies in the carburetor. The principle is to bypass air around the throttle plates, thus allowing more air in a particular throttle body. Each throttle body air compensation screw can be adjusted to create equal airflow balance in each throttle body. The air compensation screw locks in place with a lock nut.
Air Corrector Jet
The brass air corrector jet screws into the carburetor body on top of the emulsioning tubes. The air corrector jet meters air for the emulsioning tubes. There are three in each carburetor body and their size is marked on top.
Air Horn
The stainless steel air horns sits on top of the carburetor cover. The air horn acts as a diffuser, funneling air into the carburetor throttle bodies with a uniform flow. The air horn is attached to the top of the carburetor cover using a stud and nut configuration.
Auxiliary Venturi
The auxiliary venturi delivers fuel from the main circuit into the throttle body. It sits in the throttle body just above the choke and has a fuel passage way from the carburetor body to its center where fuel exits into the throttle body.
Carburetor Body
The carburetor body is the main part of the carburetor that contains the throttle bore and all working parts. The carburetor cover, 
throttle bodies, fuel passageways, fuel float floats and bowls,fuel jets, accelerator pump etc... are all either in the body or attached to it. The material is cast alluminum alloy and is rumored to be turning into gold.
Carburetor Cover
The carburetor cover sits on top of the carburetor body with a gasket in between the two and is attached using using nuts on studs from the body. The carburetor cover contains the brass fuel inlet connections, two fuel needle valves, needle seats, and needle seat plugs.
Choke
The choke is set in the throttle body and controls the amount of air entering the intake. As air flows throug it a low pressure zone is created within the throttle body which draws the emulsified fuel from the auxiliary venturi. The Weber chokes are part of the modular design, so they can be changed for another size to properly tune the carburetor for a particular application. The chokes are held in place with a fixing screw and safety wire.
Emulsioning Tube
The brass emulsioning tube aerates fuel traveling in the main circuit. It is held in place by the air corrector jet that screws in on top of it in the carburetor body. Air flows through the air corrector jet then into the emulsioning tube where it mixes with metered fuel from the main jet. The aerated fuel/air mixture is drawn into the auxiliary venturi by low pressure. There are three emulsioning tubes in each carburetor body.
Float
The float is a bulbous shaped air filled brass container that floats in the fuel bowl. It modulates up and down with fuel level. As the float moves, it opens and closes a needle valve which allows fuel into the float bowl. 
As the float lowers, it opens the needle valve letting more fuel into the float bowl. The float rises and closes the needle valve when the desired fuel level is reached in the float bowl. The float adjustment is critical to proper carburetor operation. There are two floats in each carburetor body and are held in place with a fulcrum screw and safety wire.
Idling Jet
The idling jet is a brass fuel metering device with small passageways restricting fuel that flows through it. It is held by a brass carrier that screws into the carburetor body. After the idling jet, the fuel travels to the idle mixture screw. There are three in each carburetor.
Idle Mixture Screw
The idle mixture screw is a needle valve located at the base of the throttle body that fine tunes the idle circuit mixture. Turn the mixture screw in to lean the mixture, turn it out to enrichen it. It is held in adjustment by a spring.
Idle Screw
The idle screw controls throttle valve position by rotating the throttle shaft to obtain the desired idle speed. It can be adjusted using a screw driver or by hand. Note: Never operate the engine with the idle screw out. This will let the throttle plates sit on the throttle body inner walls and will ruin the carburetor by creating grooves in the throttle body wall.
Main Jet
The main jet is a brass fuel metering device with small passageways restricting fuel that flows through it. It is held by a brass carrier that screws in at the bottom of the float bowls on the carburetor body. The main jet meters the fuel before it travels to the main well/emulsion tube.
Needle Valve
The needle valve opens to let fuel into the carburetor float bowl and closes to keep fuel out. It screws into the carburetor cover and is actuated by a tab on the carburetor float. There are two in each carburetor cover.
Throttle Body
The throttle body is a cylindrical passage way for air intake through the carburetor bore and contains the choke, auxiliary venturi, and the throttle valves that modulate air intake. The Weber triple-barrel carburetor has three throttle bodies in each carburetor body.
Throttle Shaft
The throttle shaft is a two-piece shaft that runs through the base of the carburetor body and rotates to actuate throttle valves. The throttle valves are attached to the throttle shaft by two screws. The throttle shaft also actuates the accelerating pump via cam and shaft arrangement.
Throttle Plate
The throttle plate is a circular plate attached to the throttle shaft in each throttle body. When the throttle is at idle, the throttle plates are nearly closed in the throttle body. As the throttle is opened, the throttle plate influences the idle circuit while moving past the idle progression holes in the throttle body. When the throttle is completely open, the throttle plate sits vertically in the throttle body allowing maximum airflow.
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Porsche 911 Performance Handbook 