The early A&M mechanical advance, copied by almost everyone, was a "Rube Goldberg" system that has unnecessary bell cranks and links..All these parts can become loose and sloppy(or dirty and tight) and cause uneven timing..The advantage of Syncro Spark is elimination of all these moving parts, and ease of installation on engines(mostly twins) that don't have a direct path from the carb to the moveable ring on the front of the engine..
There is only one reason for the links and bell cranks between the carb and the moveable ring..Engine manufacturers are too tight or lazy to make a carb insulator block to rotate the carb. and a new arm for the throttle shaft..
Horizon, after years of selling engines, has come up with a revolutionary new piece of bakelite to do exactly this..(at the owners' expense, of course)...My first conversion, in 1987, had this, still do, on all engines....RC.....