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Mikuni Carburetor Part No. 21100-13040

We know that on the left side is the primary Venturi where air and fuel mix together. On the right side is the secondary Venturi. What is the role of the secondary Venturi?

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The secondary venturi act as a second carburetor when there is a demand for more fuel. When an engine is designed the carb must be large enough to flow enough fuel and air for the entire RPM range. A single barrel (venturi) carb large enough for the higher RPM range may flow inefficiently at low RPM. A carb small enough to flow efficiently at low RPM may run out of fuel at a higher RPM. To get around this multiple barrel carbs were designed. The primary side allows for economy and efficiency at low RPM. The secondary side only flows fuel when the demand for more fuel is needed. The secondary side must be adjusted correctly so that it doesn't add the extra fuel to early. The result would be a over rich condition and a loss of economy and performance. The most common method to control the secondary is by vacuum or linkage. In a vacuum control a diaphragm keeps the secondary closed until the engine vacuum is low. This would occur when you load the engine (aka pedal to the metal) high rpm etc. Mechanical linkage type are dependent on how far open the throttle. As you depress the gas pedal, at a point the secondary side will progressively open.

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