Throttle valve unit

The throttle flap for a motor vehicle internal combustion engine exhaust duct has a housing (1) with a pivot shaft (5) on which is mounted the throttle flap (6). The shaft is connected to a vacuum chamber (14) with a diaphragm. The flap has an axially operating stop to define its open position, with a fixed stop (18) on a mounting plate engaging a stop (20) mounted on the flap shaft. The shaft also has an axially fixed stop ring (9) and an axially movable stop (11).