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Weber Theory

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To show the adaptability of the Weber carburetor, look at the different engines that utilize the same basic carburetor. The early Porsche 911s had much smaller engine displacement compared to the engines of today. For instance, the early 911 had a 2.0 liter engine, yet they use the same carburetor as a 2.7 or even a 3.0 liter engine does. How is this possible?

choke The modular design of the Weber carburetor allows this. The Weber 40 IDA-3C incorporates varying size chokes (venturis) in the main carburetor body that can be changed out for another size if necessary. Part of the modular design works like this: if less air is required for a smaller displacement engine, use a choke size with a smaller inside diameter, thus limiting air delivery to the appropriate CFM. A larger displacement engine would require using a larger diameter choke, allowing more air into the intake. The choke size is only one of many changeable features incorporated in the Weber carburetor.

Here is an example: For a Porsche 911 2.2 liter engine, one would select the Weber 40 IDA 3C. That means that the main body throat bore is 40mm. The choke (venturi) size fine tunes the air charge (CFM) traveling through the carburetor body. For this 2.2 liter example, choose a 30 or 32mm choke. After air metering the Weber triple-barrel carburetor provides for fuel metering using the idle jet 55, main jet 130, air corrector jet 180, emulsion tubes F3, and finally an excelerator pump. Each component can be adjusted to suit performance and tuning desires, even the acclerator pump.

Weber Normal operation

Each cylinder bank has its own carburetor. The triple-barrel carb is mounted on an intake plenum for the corresponding bank of cylinders. Each triple-barrel carb assembly has two float chambers and one acclerating pump. Carburetor operation utilizes 3 main circuits.

Weber Circuits

The Weber carburetors have 3 main circuits that deliver the fuel according to regime of engine operation. The Idle circuit, the main circuit, and the accelerating pump circuit.

Idle Circuit

The idle circuit operates from Idle to approximately 2800-3000 RPM. This circuit is adjustable by idle jet size, idle mixture screw, and indirectly by idle speed adjustment.

idle circuit

Fuel passes from the emulsion tube well through a delivery port into the idle metering jet where it mixes with air from the idle air bleed (the idle air bleed is drilled into the carb body and cannot be adjusted). The fuel/air mixture passes through the delivery port to the idle discharge port located below the throttle valve. Idle mixture is adjusted and fine tuned using the tapered idle mixture adustment screw. Just above the throttle valve are two transition port holes which begin discharging additional fuel/air mixture when the throttle plate moves by each transition port.

Note: The idle mixture screw seats are fragile and the idle mixture screw should never be forcefully closed.

Additionally, there is an air adjustment screw for each throttle body to ensure equal air flow through each carburetor throat at idle. The tapered air adjustment screw controls the amount of air bypassing the throttle plate through a passage starting above the throttle valve and exiting below.

Main Circuit

The main circuit operates after transition from the idle circuit at approximately 2800-3000 RPM to maximum engine RPM. The main circuit is adjustable by main jet, air corrector jet, and emulsion tube size. The main circuit feeds a fuel/air mixture to the throttle body through the auxiliary venturi.

Fuel flows in the main circuit from the float chamber through the main metering jet into a port that feeds the emulsion tube well surrounding the emulsion tube. Air enters the emulsion tube via the air corrector jet at the top of the emulsion tube then mixes with fuel from the emulsion tube well. Aerated fuel flows out of the emulsion tube's holes through the mixture delivery port in the auxiliary venturi. The emulsified air/fuel mixture is drawn into the auxiliary venturi by low pressure within the throttle body. Bernoulli's Principle explains the low pressure within the throttle body.

Accelerating Pump Circuit

Last, but not necessarily last in operation is the accelerating pump circuit. The accelerating pump's job is to eliminate hesitation during a large throttle increase.

As the throttle shaft rotates in the carburetor body, it acts on a shaft and roller cam arrangement that ultimately depresses a plunger in the accelerating pump. The fuel is distributed from the accelerating pump through ducts leading to a check ball at each pump nozzle. The delivery nozzle is positioned to squirt fuel into each throttle body throat. The check ball only allows fuel to travel out of the delivery nozzle, while at the same time, it holds fuel ready at the nozzle for instant delivery when called upon. The accelerating pump quantity delivery is adjustable with a small nut on the actuating cam's threaded shaft.

Weber triple-barrel >> carburetor theory >> carburetor glossary >> carburetor tuning >> jetting chart