[1QuickS]

If it was #3 or #6 I would be suspect of a broken throttle shaft coupling.

Did your service provider flow check throttles on the bench to confirm throttle valve sealing? It is easy to slightly misalign a throttle valve and cause this issue. Another item of concern following a re-bushing effort is that the couplings need to be rotated 45 degrees from their original installation position on the throttle shafts. If not rotated to a new position then the setscrews will seek their original alignment divots which will upset throttle valve alignment.