The exhaust valve opens to push out the burnt gases: it is the hottest part of the engine and wears faster than the intake.
Here you will find valves, guides, springs and stem seals for YAMAHA quads: YFM 700 R RAPTOR, YFM 350 BIG BEAR 4WD, YFM 350 GRIZZLY 2WD, YFM 350 GRIZZLY 4WD… A pitted or bent valve, or one whose clearance keeps closing up, loses compression: the engine is hard to start when hot and loses power.