Synchronizing a carburetor assembly means all four slides open at the same time. There is no "main slide" only "a slide" in each carburetor. The smoothbores, like all VM series carburetors are happiest when synchronized while the bike is running using a manometer. If you are trying to bench synchronize these carburetors, bend back the holding tab on the hex head slotted screws in front of the sync screws and loosen the hex head slotted screws so the sync mechanism is free to move. Then loosen the locknut and adjust the slide height using the sync screws. There is probably a spec somewhere for setting these where they are barely open... like .5mm or something along those lines but I like to use long heavy-duty paper clips under each slide and open the slides with the idle adjust knob and then close it down on the paper clips. Then I like to adjust the slide height so that when pulling out a paper clip, each has the same feel when pulled. This adjusts the slide height at about 1mm but the slide can be closed again when the paper clips are out of the way. You can use bigger tools to set the height up to a point. At that point, the slides will be adjusted too high across the board and the idle adjust knob won't be able to shut the slides sufficiently to get idle down.
