Three words sum up exhaust valve maintenance: spoogey, gooey, and grungy. If two-stroke exhaust valves didn't have such a dramatic effect on the engine's powerband, I'm sure mechanics would remove them and beat them bits with a hammer in frustration because there is little information given by the manufacturers on how to diagnose and repair the exhaust valve systems on well-used dirt bikes. This section is a guide to the characteristic mechanical problems that occur to the exhaust valve systems of dirt bikes. Plus we'll give you some tips on how to re-time exhaust valve systems.

An exhaust valve system is designed to increase the engine's low-end and midrange power. There are three different designs of exhaust valve systems. The first-generation design uses a variable-volume chamber mounted to the head pipe to change the tuned length of the head pipe. A butterfly valve is used to separate the surge chamber and the head pipe. At low rpm, the valve is open to allow the pressure waves in the pipe to travel into the surge chamber and effectively lengthen the pipe and reduce the pressure wave's magnitude when it returns to the exhaust port. These systems were primitive and not very effective on 125cc dirt bikes. Honda and Suzuki used this type of exhaust valve system in the mid to late 1980s.

The second-generation design features valves that control the effective stroke and the time-area of the exhaust port. These valves are fitted to the sub-exhaust ports and the main exhaust port. The main exhaust-port valves operate within close proximity to the piston to control the effective stroke of the engine. The effective stroke is defined as the distance from TDC to when the exhaust port opens. At low rpm, the engine needs a long effective stroke, which results in a high compression ratio. At high rpm, the engine needs a shorter effective stroke, longer exhaust duration, greater time-area, and a lower compression ratio. Yamaha used this system starting in 1982 on the YZ250. Honda's HPP system is similar and was used on the 1986–91 CR250 and 1990 to current-model CR125.

The third generation of exhaust valve systems attempts to change the exhaust-port velocity, effective stroke, exhaust-gas temperature, and the pressure of the compression wave. Yamaha and Suzuki started using these systems on their 125s in 1995. Both companies employed a venting system to the outside atmosphere. This is very complex because they are attempting to affect the temperature and pressure of the returning compression wave to synchronize it with the piston speed. The exhaust-gas velocity and the effective stroke are controlled by two oval wedge valves that enter the exhaust port at a 45-degree angle. The wedge valves partially block the exhaust port, thereby boosting the gas velocity. Kawasaki's KIPS system uses wedge valves in the main exhaust port to control the effective stroke, drum valves in the sub-exhaust ports to control the time-area, and a surge chamber to absorb the excess compression-wave pressure at low rpm.

The exhaust valves are opened and closed by a centrifugal governor mechanism. The governor is mounted under the right side cover and is gear-driven by the crankshaft. As the engine rpm increases, the governor spins, thereby increasing the angular momentum of the four steel balls encased in the governor. The steel balls fit into an angled ramp-and-cup arrangement. A spring is used to provide tension on the steel balls. When the momentum of the steel balls overcomes the spring's tension, and the balls force their way up the angled ramp. A spool attached to the ramp, enabling it to change its linear position with changes in rpm, and the spool is attached to a linkage system that operates the exhaust valves in the cylinder. Factory race teams have different combinations of springs, ramps, and balls to tune the exhaust valve operation and enhance the powerband.

Although exhaust valves use the same essential principles, the implementation is different with each manufacturer. Also, each type has its own flaws and fixes. The list below gives you tips on how to install and service the most common exhaust valves, as well as some tuning tips