Oh yeah, I forgot that the Saito 100 has a pressure tap on the muffler.

That is where the vent line from the tank connects. The exhaust pressure pressurizes the ful tank.

You can use a 2 line system and just put a Tee in the fuel supply line from the tank to the carburetor, but the fuel tends to pour out of the carb when you are filling unless you use a pair of hemostats to pinch the fuel line at the carb while youa re filling. The 3 line system is a much neater way to fuel the engine.