It's a Y.E.I.S or more commonly called a Boost Bottle. YAMAHA ENERGY INDUCTION SYSTEM (Y.E.I.S, "Boostbottle") Y.E.I.S is a very efficient invention. A small bottle, often called "boostbottle", is installed between the carb and the intake port connected with a branch pipe to get the engine to respond quickly from bottom end without flat spots. It saves 10-15% in gas consumption and makes the engine wake up at lower rpm. Like many of Yamahas inventions. When it first came out there were many doubters. Just as they doubted the wisdom of a separate oil pump and the 7th port. Or Torque Induction. But if done properly this does work. Since Carburators can carburate both in and out, when the charge is out of phase with the engine it creates a double dipping intake charge (passing the jets twice). This kills the engines ability to make Torque due to this RICH condition, thus creating the Torq\HP DIP just before the engine starts to take in the charge and use it all. This Power dip can be seen in both 2 and 4 stroke engines. The ~BOOST~ Bottle will help cure the problem by providing a storage tank, of the required volume and distance to resonate at this RPM. It stores the charge that once reversed back through the carb and returned as a DOUBLE RICH condition.
