The intake valve opens to let the air-fuel mixture into the combustion chamber, then closes to hold compression.
Here you will find valves, guides, springs and stem seals for YAMAHA quads: YFM 700 R RAPTOR, YFM 350 GRIZZLY 2WD, YFM 350 GRIZZLY 4WD, YFM 350 WARRIOR 2WD… 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.