There are no "air/fuel mixture screws" on the 650 throttle body, there are only two synchronizing screws are used to align (synchronize) the throttle plates.
I did this on a friend's '09 650; it is done by setting the idle to 1200 rpm using the idle stop screw, and then adjusting the synchronizing screws to get the same manifold vacuum in each cylinder. The adjustments are a bit interactive, I.e. the idle speed may change while adjusting the sync screws, if it does return it to 1200 rpm and recheck the vacuum balance.
In the last set the idle to 1200...