This O Ring should be placed on the Idle Mixture Adjusting Screw with a Special Cup Washer (also available here) used to keep the O-Ring in place against the Side of the Carbs. These O-Rings and the special washers can make a huge difference in the way your Porsche runs! Without the washers and o-rings the engine will have a vacuum leak and it will back-fire, spit and sputter except when accelerating (which is usually very smooth). The O-Rings are Item # 43 on most Weber 40-IDA parts diagrams.