very interesting. so running 2 carbs? excuse my noobness but i thought that when your cyclenders fire it fires 1&4 and 2&3 so if you had one carb for the 1&2 and 3&4 wouldn't it only be shooting fuel in on the 1&4 rotation? 
1&4 fire together and so do 2&3 because the ignition is a waste spark system. When 1 is finishing its compression stroke and firing its plug, 4 is finishing its exhaust stroke, and vice-versa (and same with 2&3).
So at any given moment, exactly 1 of the cylinders is in each of the 4 strokes.
Now, the spark timing really has very little to do with the carbs. When you talk about a carb "shooting fuel", this sounds a lot more like a sequential fuel injection setup. The way that a carb works is VERY different. With a carb, air is flowing through in an almost continuous but complicated pulsed pattern with air flowing in and pressure waves flowing in and out. The movement of the air through the carburetor uses a variety of metering schemes and fluid dynamics effects to provide the correct amount of fuel for the air based on throttle opening, various pressure differentials, etc. It doesn't really "shoot fuel" the way an injection system does.
In a typical setup for an I4 engine, you run 4 single-barrel carburetors with a common throttle linkage. In this case, it's 2 double-barrel carbs. You'll see that each cylinder still gets it own venturi, so it's effectively equal to the setup with 4 double-barrel carbs. If he was running 2 single-barrel carbs, then it would make more sense to pair 1&4 and 2&3, because that would result in evenly-spaced intake pulses, but that would create significant packaging problems since you'd need a mess of plumbing up in the front of the engine behind the front wheel.
This is all a bit disjointed, but hopefully it makes a bit of sense.
