Revival of the well tried Venturi air systems from Doppler with a new design and a transparent cover. It utilizes the so-called venturi effect. What does that mean? The idea is that by reducing the effective flow-area in precisely defined places, one also reduces the suction needed to start an airflow. Just think of different straws, one with too small a flow area (diameter of say half a millimeter) you will find it difficult to suck your drink through, with a straw of a diameter of 5 cm again you will notice that suction poses problems. Flow areas too small as well as those too large require loads of suction forces, and that's something our 70cc engines can't really supply by the truckload. Doppler has set out to solve this problem, their filter comes with two inserts to adjust it to the use with any carb between 12mm and 21. And though it has not been mentioned yet: the venturi design also helps to keep the noise down as much as possible. Fits all Dellorto 12-21mm carbs and all Arreche 12-24mm carbs.