Valve Burning is sometimes attributed to the use of leaded fuels although there is little justification for this view. The carbon produced by leaded patrol is much harder than that formed when a normal fuel is used and is of grayish colour.
As these fuels usually contain a red dye, the deposit produced can be mistaken for burnt metal: but if the deposit is removed experimentally by boiling the valve in the a strong solution of caustic soda a clean metal surface will usually be found beneath it, a further reason for attributing valve burning to the use of leaded patrol is the fact that the improved anti-knock properties of these fuels allow engines to be run for greater periods between de-carbonization, whereas had the engines been de-carbonized and the valves ground in at an earlier stage, no sign of excessive pitting or burning would have been discovered.
When premature valve burning cannot he attributed to any of the foregoing factors. It should be verified that the valves are manufactured from a suitable grade of steel. Various alloy steels are used for the inlet valve including plain nickel steel, nickel chromium and chrome molybdenum. Valve Breakage The steels employed for exhaust valves include nickel chrome high tensile steel (known as austenitic steel) and tungsten steel) and tungsten steel. Valve breakage is comparatively rare but it can cause severe damage to an over-head-valve engine owing to the valve following into the valve following in the combustion chamber and striking the piston. Usually the trouble can be traced to the formation of a crack, which will rapidly spread as the result of very high stresses and temperature to which the valves are subjected.
A common cause is over speeding of the engine, ladling to valve bounce, which can also be caused at normal speed by weak springs or spring surge. If the clearance between the valve and the guide is too great, the valve head may be subjected to lateral loads. Excessive tappet clearance can impose high impact loads are the result of increased opening and closing velocities. Bent Valve’s Stem A bent valve’s stem will also cause the trouble. Excessive valve temperature can result from the accumulation of sludge or scale on interior surfaces of the cylinder head or block. The valve sticking can cause burning of valve seats. No valve material can withstand indefinitely the blow-torch effect of the exhaust flame playing on the seating. Burned Valve A burned valve results when the heat form combustion blows away a small portion of the valve face. This allows the pressure to look out of the combustion chamber and enter the intake or exhaust part. The air fuel mixture will not ignite and burn. The engine will miss especially at idle. With a burned valve you may be able to hear a puffing sound as pressure blows past the valve. There may be a popping sound at the carburetor or through body (bad intake valve) or at the exhaust system tailpipe (burned exhaust valve).
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