X2 that and he would know as he is 'the' SOHC guy here.
Sorry for the confusion on syncing. They ALL set mechanically as that is the only physical way you can do it. What I meant was by using like very small drill bits say 1/32" in all 4 bores, carefully done it will sync mechanically as well as any other method can, the slides will move dead evenly.
However, as engine gets older you may not want or need that as the individual cylinders will begin to wear in different ways and then sometimes you want say one slide to be up a bit higher to begin with or lift at motion a bit faster in an effort to make them all closer together. Done by vacuum only then, a pure mechanical method will not work there. In effect you are mis-syncing slightly to bring the 4 cylinders in closer together in realworld action due to wear influences. Needed say for one cylinder not seating as well at the valves to then need to lift fractionally quicker to stay up with others, an example. I have a CB550 that requires that, it has a lot of wear and time on it being an all original bike from '77, but play with it long enough and it gets to where it pulls off idle really well, but not easy to get there. The slides actually get somewhat uneven there but the bike pulls off idle very well like that. If you set them all dead even (mechanically, not by vacuum) it pulls away doing all sorts of funny things, you can hear the different cylinders fighting with each other as one comes on later or earlier than the others. That does not happen on new engines though, they run fine set like that.
Since carb bank is off engine at rebuild time I tend to mechanically preset the slides and then watch what the engine wants once it carboncoats to begin to run like it will longterm. If error shows up then I set by vacuum.