That is set by the idle screw and all three are sync'd to the master. Easy way to sync carbs is to take a drill bit, i use a 1/4" and adjust the idle so you can just slide the drill bit under the master valve. (cant remember which ones the master off top of my head) When you take the 3 carb tops off you will see that one is put in with a set screw and no adjuster, I/E master. Then adjust the other two to be the same. Then just readjust idle to your liking when you put all back together and fire it up. I keep my idle around 1200 to get rid of idle hang so my gap(a) will be different then someone who sets the idle at 1800 which is stock. Not sure you wanted to know how to sync carbs but kinda explains what the gap (a) is.