Valve lash is one factor which determines how far the intake and exhaust valves will open into the cylinder. All vehicles covered by this information are equipped with mechanical valve lifters, which must be adjusted at the factory recommended intervals (1970-77, every 12,000 miles; 1978-84, every 15,000 miles; 1985-87, every 30,000 miles).
If the valve clearance is too large, part of the camshaft lift will be used to remove the excessive clearance, thus the valves will not be open sufficiently. This condition has two effects, the valve train components will emit a tapping noise as excessive clearance is taken up, and the engine will perform poorly, since the less the intake valve opens, the smaller amount of air/fuel mixture is admitted to the cylinder. The less the exhaust valves open, the greater the back-pressure in the cylinder which prevents the proper air/fuel mixture from entering even if the intake valve did open properly.
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 accomplishes 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 efficiently 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