The big belcrank doesn't reallyhave it's own stop apart from the the big idle knob on the right hand side. The fats idle cam will hold it open farther with choke application.
As to actual idle speed, that is controlled with the throttle valve opening. The big idle stop knob should both allow the slides to completely bottom in their bores, and have adjustment to hold them open enough to allow a low idle (after warm up).
If it is not coming down in RPM enough, it could be carbs slides out of sync, which will make the cylinders fight each other for dominance of crankshaft rotation speed. However, turning out the big idle knob should allow a near total shut off of air supply to the cylinders, provided the choke mechanism is not actuated and the fast idle screws is not set to ridiculous positions.
Certainly air leaks will mess up idle, as well.
I'd set your pilot screws to 2 turns with today's ethanol fuel contamination. You can fine tune them inward later for a leaner idle, or tolerable response to the throttle twist.
Cheers,
