I’m not sure if you’re talking about a “certain” position to synchronize the
carb butterfly / throttle plate with the magneto, or elsewise.
I have to think it’s purpose is just like any other engine to that uses throttle
plates in the fuel system, to regulate air flow through the carb, and hence
atmospheric pressure and fuel delivery.
If I could see a picture of how the spring is set and the throttle plate before closing it up, that would be a great help. Someone had the motor appart and now I’m not sure what’s right.
At one time, there must have been a lever, or gear like mine,
attached to your broken throttle shaft.
Regarding the spring, it’s probably bent, or the end broken off.
One end is in a hole in the throttle shaft, and the other end
must have hooked on the carb body some how.
Hopefully someone will come along with better photos or knowledge.
Keep an eye out on Ebay for Flambea parts or photos of carbs.
Dumb me, I went and looked at the cover I had primed and there it was a manual working lever for the butterfly! You push it in and turn, I should have remembered that when I took it apart.