Why does the Rotax's Bing 64 use a starting carburetor instead of a real choke?

The Rotax rotary valve twin cylinder two stroke engine utilizes 2 carberators, one carberator dedicated to each cylinder. Each carberator has an enriching circuit that adds additional fuel for cold start situations, the same function that a butterfly choke achieves. The enriching circuit style has the benifit of no additional parts in the airflow through the venturi, making it safer and more efficient. 

Dennis Richardson wrote:

The Rotax rotary valve twin cylinder two stroke engine utilizes 2 carberators, one carberator dedicated to each cylinder. Each carberator has an enriching circuit that adds additional fuel for cold start situations, the same function that a butterfly choke achieves. The enriching circuit style has the benifit of no additional parts in the airflow through the venturi, making it safer and more efficient. 

I'm talking about the Rotax 912ULS, which is a four stroke four cylinder engine.

I also don't know if the "enriching circuit" that you describe for whatever two stroke engine you're talking about is the same thing as the starter carburetor. Does it use the Bing 64?

Yes, the so-called "starting carburetor" is more properly called an "enriching circuit".  Instead of having incoming air being restricted by a separate "choke" valve upstream of the normal fuel jets (with the throttle open), the air is just restricted with the throttle valve, with the fuel coming through this separate circuit instead of the usual jets.  The extra vacuum from the closed throttle allows the enriching circuit to deliver the extra fuel – it will only really work, when engaged, if the throttle is closed.

Safety. Suppose you are full power on takeoff. Your hand is on the throttle, as it should be. Your girlfriend's little brother, who is riding shotgun decides to pull (or push) on the choke (or enrichener) knob. What happens if it is a standard choke? What happens if it is an enrichener circuit?

Tyler Said:

– it will only really works, when engaged, if the throttle is closed.

And there is the heart of the matter.

Discounting that BING designed the Carbs and not Rotax...

If the Choke/Starting Carb/Enriching Circuit malfunctions, the physics causes it to Fail-Safe!

If one or Both sides malfunctions, you might end up with a terrible idle but the mid and higher power levels will still be operational when you really need them.