A walk power mower has a deck that provides a cutting chamber for enclosing a rotary grass cutting blade. An assembly is provided for adding ambient air to the cutting chamber during a grass mowing operation to improve performance. The assembly comprises an air vent placed in a top wall of the cutting chamber. An external cover outside of the cutting chamber and above the top wall of the cutting chamber has an air permeable section located forwardly of the air vent to admit air into the cover to thereby feed that air to the air vent. An internal cover inside the cutting chamber and below the top wall of the cutting chamber receives the air passing inwardly through the air vent and discharges such air into the cutting chamber at a location that is spaced from the air vent.