Burned valves are indicated by irregular exhaust gas temperatures and sometimes by excessive noise. In general, the principal causes of burned valves are carbon deposits, insufficient tappet clearance, defective valve seats, and valve heads that have been excessively reground.

The principal cause of burned exhaust valves is small particles of carbon that lodge between the valve head and the valve seat. These particles come from incomplete combustion of the fuel or oil left by the piston rings in the cylinder. The particles hold the valve open just enough to prevent the valve head from touching the valve seat. The valve is cooled by several means, including its contact with the valve seat. When carbon particles prevent contact, the heat normally transferred from the valve head to the seat remains in the valve head. The valve seat seldom burns because the water jackets surrounding the seat usually provide enough cooling to keep its temperature below a dangerous point.

When cleaning carbon from cylinder heads, remove all loose particles from the crevices; be extremely careful that you do not nick or scratch the valve or seat. Removing the valves from the engine will make it easier to clean the passages and remove the carbon deposits from the underside of the valve heads.

Check the tappet clearance adjustments at frequent intervals to be certain they are correct and that the locking devices are secure. The adjustment of valve clearances is discussed later in this chapter.

Most engines are equipped with valve seat inserts made of hard, heat-resisting, alloy steel. Occasionally, a seat will crack and allow the hot gases to leak, burning both the insert and the valve. Sometimes a poor contact between the valve seat insert and the counterbore prevents the heat from being conducted away, and the high temperatures deform the insert. When this occurs, both the seat and the valve will burn; the seat insert must be rep1aced.

LOOSE VALVE SEATS

You can avoid causing loose valve seats only by installing them properly. Clean the counterbore thoroughly to remove all carbon before shrinking in an insert. Chill the valve seat with dry ice and place the cylinder head in boiling water for approximately 30 minutes; then drive the insert into the counterbore with a valve insert installing tool, as illustrated in figure 3-10. Never strike a valve seat directly. Do the driving operation quickly, before the insert reaches the temperature of the cylinder head

When replacing a damaged valve with a new one, inspect the valve guides for excessive wear. If the valve moves from side to side as it seats, replace the guides.

Figure 3-10.Driving a valve iusert into the cylinder head counterbore.