2000 King Quad 300
It's actually adjusting the tappets, but who worries about semantics?
Some ATV's are being made with hydraulic lifters these days, like Polaris twins, and some four valve DOHC machines use winkler caps with shims underneath to adjust lash, but most ATV motors still permit setting tolerance with the tappet screw.
The valve lash needs to be within a certain tolerance. Too tight and the valves leak, causing lost compression and the likelyhood of a burnt valve eventually, and too loose means again lost power, increased noise and increased wear on the tappet and valve stem.
They're adjusted correctly when you get the bike new, but operational wear in on the tappet surfaces and the cam and stem makes them looser and wear on the valve and seat makes them tighter, and the two don't always cancel themselves perfectly, so you have to adjust or at least check them periodically. Remember when the motor warms up the lash tightens up a bit. It's made to accommodate this.
You adjust the lash when the tappet is 'off the cam'. See, the valve is only <opening, open, or closing> a small portion of the time. Most of the time it's closed and the tappet rides on the part of the cam that is ground to be round, as opposed to the lobe part. Any time it's on the round and not near a lobe ramp you can set it. Top dead center of the COMPRESSION stroke (not the exhaust/intake stroke, pay attention!) is stated in manuals so that the common garage mechanic has a wide margin of safety when setting the lash, but hey guys, at the top of the compression stroke both intake and exhaust are a good 90 degrees cam rotation from lifting. You only need to be close, in piston position. No need to fiddle around getting the FT lines lined up precisely! Off the cam is off the cam. Here's how you do it perfectly, every time. No rocket science here.
Take out the spark plug, pull the motor over slowly with the starting cord with your little fingy over the plug hole, mostly covering it, and when you feel compression whoosh against your fingy then leggo the starting rope and you can set the tappets.
Anyway the tappet covers come off and you see the tappet screws and their jam nuts that keep them where they're set at. (I call it a screw but yours probably has a square head) You should be able to wiggle them a bit at this time unless they are tight as... never mind. Loosen the jam nut and back out the screw a bit. Intake side is next to the carb, get your .002 -.003 feeler gauge (looks like a spatula for flipping the worlds smallest flapjack) and slip it between the tappet end and the valve stem head. Tighten the screw on it till it slides freely but any tighter it would drag hard. Then take out the feeler, hold the screw and tighten the jam nut so the setting is preserved. Do the same on the exhaust side using the .004-.006 feeler gauge.
Put the cover caps back on. Ride.