When the engine's running, the valves move at eye-blurring speed in a delicate and precise manner. However, if the balance and precision of one of the moving parts associated with a valve train-the lifters, pushrods, springs, rocker arms, and the valves themselves-is affected, then chaos quickly ensues. The opening and closing of the valves allows fresh air to enter the cylinder and exhaust gasses to exit once combustion is complete. If the clearance between the valve stem and the rocker isn't within the engine manufacturer's tolerance, the valve will either open too far or not far enough; it may also stay open too long or close too soon. Any of these scenarios can cause decreased power, hard starting, poor fuel economy, and excess smoke.
Valve adjustment should occur at the engine manufacturer's specified intervals, perhaps every 250, 400, or 500 hours; for precise figures, consult the owner's manual for your engine. Adjustment requires a clear understanding of the procedure-experience helps, but it's not necessary, provided that you follow the service-manual instructions-as well as the ability to use feeler gauges and a wrench.