KTM uses two distinctly different designs of exhaust valves systems. The earlier model uses one large center valve with the actuating rod cast together. Two drum valves control the sub exhaust ports and steel gears are used to interface the main and minor valves. The effective stroke can be adjusted by altering a stop plate on the left side of the cylinder. The governor control in the right side case has an inspection cap that allows tuners to add thin washers and increase the spring preload in order to affect a change in engagement rpm. The system is prone to carbon seizure of the steel valves (sub exhaust ports). Also there are rubber o-rings that prevent oil from leaking out the sides of the actuating rod. Those eventually wear out. In order to service this exhaust valve system you need to remove the cylinder. The main valve and all it's hardware can remain bolted together. There is an access cover on the front of the cylinder and four bolts fasten the cover to the cylinder. There is no Gasket for the cover, it seals with a non-drying liquid gasket like RTV silicone. The main valve just pulls straight out. The drum valves are held in place by two plates with two-tapered pan head Phillips screws. There is a specific procedure for removing these screws. Start by heating the screw heads for two minutes with a propane torch. That will break down the locking agent on the threads. Then use hand impacts with a number two Phillips tip. If you strip the heads of the screws (most people do including me!) then just use a tapered-point punch to spin the screws out. That will destroy the screw but you should replace the Phillips with a tapered-panhead Allen bolt. Take care when handling the stop lever on the left side of the main valve. Before you loosen the two Allen bolts, scribe a line to reference the position of the plate relative to the gear plate. It is possible to adjust the stop plate so far that the valve rotates past the full closed position and contacts the piston. That would destroy the piston. The 1998 and newer KTMs have an exhaust valve system that makes use of the resonator concept. That system is so complex that you would need the factory service manual in order to service it.