 Sometimes one or more additional "booster venturis" are placed coaxially within the primary venturi to increase the effect.As the throttle valve is closed, the airflow through the venturi drops until the lowered pressure is insufficient to maintain the fuel flow, and the off-idle circuits take over again, as described above.Bernoulli's principle, which is a function of the velocity of the fluid, is the dominant effect for large openings and large flow rates, but since fluid flow at small scales and low speeds (low Reynolds number) is dominated by viscosity, Bernoulli's principle is ineffective at idle or slow speeds and also in the very small carburetors of the smallest model engines.