Refer to the owner's manual for vehicle maintenance schedule (usually 30,000 miles/48 months or 60, 000 miles/72 months - some 1984 vehicles 15,000 miles/12 months) for valve clearance adjustment (lash) as all years and model engines have different mile and time intervals.
Valve lash is one factor which determines how far the intake and exhaust valves will open into the cylinder. If the valve clearance is too large, part of the lift of the camshaft will be used up in removing the excessive clearance, thus the valves will not be opened far enough. This condition has two effects, the valve train components will emit a tapping noise as they take up the excessive clearance, and the engine will perform poorly, since the less the intake valve opens, the smaller the amount of air/fuel mixture admitted to the cylinders will be. The less the exhaust valves open, the greater the back-pressure in the cylinder which prevents the proper air/fuel mixture from entering the cylinder.
If the valve clearance is too small, the intake and exhaust valves will not fully seat on the cylinder head when they close. When a valve seats on the cylinder head it does two things, it seals the combustion chamber so none of the gases in the cylinder can escape and it cools itself by transferring some of the heat it absorbed from the combustion process through the cylinder head and into the engine cooling system. Therefore, if the valve clearance is too small, the engine will run poorly (due to gases escaping from the combustion chamber), and the valves will overheat and warp (since they cannot transfer heat unless they are touching the seat in the cylinder head).
While all valve adjustments must be as accurate as possible, it is better to have the valve adjustment slightly loose than slightly tight, as burnt valves may result form overly tight adjustments.