Headtube is right. All different models of Bings on many different models of BMW bikes, they all have a specific setting for the idle mix screw. This is the smaller screw on the bottom of the carbs. On your model this screw is set to One (1) full turn out. Please set this screw to ONE full turn out and leave it there.
The idle speed screws are the larger screws on the bottom side of the carbs. They can also be set to one turn in at this point. these are the screws you will use to balance the carbs and get the correct idle speed.
Use the enrichners to start the bike. Turn them part way off after bike starts. If cold out the need to be a little bit on for a little longer but if warm out or the bike is already warm they can be taken completely off sooner.
You are not supposed to idle an Airhead in a stationary place for 20 mins. If you really need to do this set up a large box fan to blow air on the bike. You are allowed to idle for only 5 mins. The fact your bike dies after 20 mins of idling is probably because it is over heated.
Once you get the bike running good enough to ride you should take it for a ten to twenty min ride to warm further. At this point you are ready to balance the carbs. Do you have the shorting tools, little studs to fit on plugs, or other balancing tool? Idle speed should not be higher than 1000 rpm in my opinion. I hear of someriders having their bikes running at 1100. Still 1500 is too high.