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The Reed Valve

    The reed valve is a device which opens under vacuum, allowing the fuel/ air mixture to enter the crank case of a two stroke engine. The vacuum is generated as the piston makes its up stroke. Then as the piston makes its down stroke vacuum in the crank case changes to pressure, forcing the reed valve closed. This keeps the fuel in the engine and aides in pressure forcing the fuel charge up the transfer ports into the combustion chamber. As the piston starts to make its up stroke the transfer ports are closed allowing the piston to compress and fire the fuel. The case reed valve works in the same way, but is located in a different part of the intake tract.
    Before reed valves the popular set up was piston port which did work, but had some issues. With no reed valve, there were occasions where fuel would get pushed back through the carburetor and end up on the air filter, which could be a fire hazard, or significantly reduce the ammout of air that could pass through the filter, resulting in poor performance. Also, when running at slow speed the engine could load up with excess fuel, also causing poor performance, and spark plugs to foul. With the reed valve, the above issues were solved, and with the valve there was also a noticeable change in the power curve of the two stroke, as it now could run cleaner from idle to peak horsepower. Although all Japanese manufacturers eventually adopted the reed valve, several after marked firms took the basic design and drastically improved it, not only in terms of performance, but with innovative new materials. Personally I use Boyesen reeds in my classics, and they are top notch.
        Yamaha was the first of the Japanese manufacturers to use the reed  as standard equipment beginning in 1974. Their design was to place the reed in the intake, just ahead of the carburetor, known as piston port reed valve. Suzuki introduced their reed valve, but placed it on the bottom of the cylinder at the crank case, known as case reed valve. Both systems worked well, but eventually, the system pioneered by Yamaha, was used by the big four. Having said all of this, the old addage, "everything old is new again" likely applies to the reed valve. In a lot of cases, what we consider as the latest in technoligical advancements, are a new take on something tried decades before that failed. Most times, these failures were the result of inferrior materials, which were later invented.
    Attached are a few drawings I pulled off the web, showing the function of the reed valve to help people better understand their function.