So not reading your service manual these comments may be all wet but when I synchronize Yamaha Carbs I have the linkage loose, Nothing touching the timing advance cam on the throttle linkage, pencils with the eraser end lightly holding the butterflies closed on all butterflies at the same time (can get tricky) I lock down the lefthand thread screw that keeps all the linkages together so all butterflies open in perfect union. Does your engine have the same adjustment as the Yamaha's I have experience synchronizing?