A throttle valve is provided within a discharge air duct located adjacent a fiber duct receiving a fiber laden air stream. The throttle valve cooperates with a stop in a lowered position to form a gap to allow a minimal air flow therethrough. In addition, a weight is provided on an arm attached to the throttle valve for pivoting therewith. The weight is disposed to counterbalance a substantial portion of the weight of the flap valve in the raised position of the valve so that the air flow maintains the valve in the open condition.