A manifold is connected between an extruder and the associated die assembly to supply plastic flow to the die. The output from the extruder is split into multiple passages in which each manifold passage supplies plastic flow to an extrusion passage within the die assembly. An adjustable throttle valve is positioned within each manifold passage to regulate the flow volume through the manifold passage, thereby controlling the output of the die assembly. An adjustment mechanism is constructed having a lever arm actuated by a screw to provide movement of a spring biased part of the valve to open and close a valve seat against a valve stem.