Follow the sequence to the letter. First of I think is the rear left, it doesn't have a sync screw and is only adjusted to set the idle and that would be the 'hidden' thumb screw between the 2 carbs and is your base carb.. Set that to 900rpm, then the other carb on the left side needs to pull the same vac. It seems you have that about right. Always keep the idle at 900 or you will change the vac draw as you go, and that is done by adjusting the idle screw only on the left side. Now comes the fun part on the right side, which screw does what. Look closely at the linkage, one carb is linked to the left side, the other has a link going to the other one on the right side. First sync the right side together, keeping the idle at 900, then adjust so that the right side 2 come together with the left side, always going back to adjust to 900rpm.