Flow control device of a helically-shaped intake port

A helically-shaped intake port comprising a helical portion formed around an intake valve, and a substantially straight inlet passage portion tangentially connected to the helical portion. A bypass passage is branched off from the inlet passage portion and connected to the helical portion. A rotary valve is arranged in the bypass passage and actuated by a vacuum operated diaphragm apparatus. The rotary valve is opened when the amount of air fed into the cylinder of an engine is increased beyond a predetermined value. The diaphragm apparatus comprises a diaphragm and a control rod interconnecting the diaphragm to the rotary valve. A first stop and a second stop, which are engageable with the housing of the diaphragm apparatus, are fixed onto the control rod for retaining the rotary valve at the full open position and at the completely closed position, respectively.