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Two-Stroke Top End Rebuilding
By Eric Gorr
Yamaha Powervalve
Yamaha was the first motorcycle manufacturer to adapt exhaust valves to two-stroke motorcycle engines. Yamaha's simple design of a cylindrical valve that rotates 1/4 turn to vary the height of the exhaust port requires little maintenance. This system was used on the YZ250 from 1982-98, and on the YZ125 from 1983-93. Occasionally, you have to replace the seals and O-rings to prevent exhaust oil from drooling out of the side if the cylinder. In 1989, Yamaha added a stop plate to limit the travel of the power valve, primarily so mechanics couldn't install the valve in the wrong position. The stop plate is located on the left side of the cylinder. The valve has a small tab that bumps up against the stop plate to limit the fully open and closed position of the valve. This design enabled Yamaha to position the valve closer to the piston to make it more effective at varying the exhaust-port timing. Unfortunately, the soft-aluminum tab on the valve gets worn, allowing the valve to rotate farther in the fully closed position. Eventually, (after about three years' use) the tab wears enough so the valve strikes the piston, causing damage to the piston. Yamaha's exhaust valve is cheap to replace. I recommend replacing the valve when the tab wears more than 0.030 inch (0.7mm).
In 1994 Yamaha changed the engine design of the YZ125 and included the next generation of exhaust valves. This system used two oval-shaped wedge valves, positioned at a 45-degree angle over the exhaust port. This system was similar to the one employed by Suzuki. Yamaha experimented with resonator cavity volume, and vents for pressure bleed off and temperature control. Overall this is a very reliable system. Occasionally the pins that fit through the ends of the valve to interface with the actuator lever vibrate out causing the valve to strike the piston. Those pins are a press fit but you can add some Loctite Instant Adhesive to the pins for added protection. One problem that Yamaha is concerned with is high rpm valve flutter. They've added springs to the valves to control the flutter but future innovations could include a positive seal between the valve and the cylinders' valve pocket.
In 1999 Yamaha redesigned the YZ250 engine and exhaust valve system. This model features a powervalve that marks a significant design change, from the company that pioneered the use of exhaust valves on two-stroke engines. Looking more like a Rube Goldberg device, the new powervalve has separate valves for the main (center) and sub-exhaust ports (sides). The whole assembly is controlled by one actuating rod, but the side valves open after the main exhaust valve. The side valves are controlled by two wedge-shaped ramps that resemble a shift drum from a transmission. The ramp design offers versatility in tuning. By changing the shape of the ramp, the duration and timing of the sub exhaust ports can be changed to match a rider's ability or the demands of the terrain. So far there are no aftermarket companies making these ramps but you can bet that the factory teams are experimenting with them!
The new exhaust valve system also features an atmospheric vent that not only relieves the pipe pressure wave at low rpm but also introduces cool air into the exhaust system to make the pipe work over a larger rpm range.
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