So Fix it

Reduce the amount of fuel injected and cause the mixture to go lean. Lean air/fuel ratios are probably the single most common culprit behind burned valves. Engines arent designed to burn all of the fuel they inhale; a large portion of it exits the cylinder unburned. Leaning out the air/fuel ratio allows more of the fuel to burn, which increases power and fuel economy (to a point), but it also increases the temperatures inside the cylinder. A clogged or malfunctioning fuel injector can also cause a cylinder to go lean, as can an improperly-tuned computer or carburetor.

Slide a huge camshaft into the block. After the exhaust valve picks up heat from the combustion chamber, it has to shed the heat into the cylinder head. A certain amount of this thermal transfer occurs through the valve stem, but much of it takes place while the valve remains shut against the valve seat. A big camshaft keeps the valves open longer, decreasing the amount of time they spend in contact with the cylinder head, and thus the amount of thermal energy they can shed during each cycle.

Dont install an exhaust manifold or exhaust headers. Old-school wisdom may have it that an engine needs exhaust back-pressure to function, but this is about as true as saying that people have to hold their breath to live. The reason that engines need an exhaust manifold is that the exhaust valve stays open after the piston starts moving downward in the cylinder. Without a manifold, the exhaust gases would shoot out and the piston would suck oxygen in from the outside and into the chamber, causing the mixture to go lean.

Advance your timing excessively. Odds are that excessive ignition timing will result in premature detonation (knock or ping) before it does a burned valve, but sustained high-rpm usage combined with excessive timing can easily damage the valves. This is especially true if you combine a bit of extra timing with a large cam and a lean air/fuel ratio, as some hot-rodders are apt to do.

Allow your engine to overheat and wear out. All the valve-to-seat contact time in the world wont save your valves if the coolant is either too hot to carry away thermal energy from the head or is absent entirely. Worn valve guides are another potential trouble spot, since the valve stem needs constant contact with the head in order to remain cool.

Drive your car with a malfunctioning oxygen sensor. This can cause thermal failure, which means that excess raw fuels are making their way, in large quantities, to your catalytic converter.

Drive around in a car that has a starter or engine that misfires. This can also cause thermal failure of the catalytic converter.

Drive around in a car with old spark plugs that are prone to misfiring. This too can cause burnout.