 Without the port valve, the long induction tract of the secondary port would fill with traces of exhaust gasses whenever the secondary valve was closed.This occurs as the tip of the rotor passes the induction port at the end of the exhaust cycle before beginning the induction cycle.If this was allowed to happen, when the secondary throat finally opened the engine would first swallow an induction charge contaminated with exhaust gasses causing a momentary misfire and felt as a dead spot or hesitation in acceleration.