Will a WB boost bottle work on a 400EX?
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Boost bottles are only for 2-stokes. As the intake is sucked into the crankcase, it creates a low pressure area inside the bottle. The low pressure then pulls air into the bottle while the reeds are closed. As the crankcase pulls air in, it is taken out of the bottle as well as the intake (increases intake volume just as Gary said). They are not only for twin cylinders. On twins they are supposed to help equalize pressure between the two cylinders. Many yamaha's had boost bottles from the factory, old YZ250-490, Tri-Z's and I believe early blaster's. Gary is right, they really don't work.