To vacuum sync them, the carbs have to be on the bike and the bike running. You then remove a philips screw in the intake runners and insert a fitting to attach a way to measure vacuum. You would do this on both sides at once.
In between the carbs, but attached to the left carb there is a set screw and a locknut on the linkage connecting the carbs. If not ever messed with, it may have a drop of yellow paint on it. By adjusting this screw. the vacuum can be changed so that both carbs are the same. This allows each cylinder to equally share the load. In theory, this would also be the spot that each carb's butterfly is equally opened.
This all sounds very cut and dried, but in real life, there are compromises. Getting the vacuum exactly the same can be a frustrating challenge. It will want to vary when you tighten the screw, and then not return to equal when you blip the throttle. And tightening the locknut will also change the setting. The adjustment screw also is difficult to access with the engine running and the RPM's also want to change depending on how hard you are pressing down on the screw with your screwdriver. Practice improves your success rate tho. I try to get within 1 mm of Hg if possible, but 2 mm is more doable.
