Engine Valves - Some Helpful Information
Engine valves control the flow of the intake and exhaust gasses through the cylinder head and to/from the combustion chamber. The complete cycle of intake, compression, firing, and exhaust takes place in less than 1/25 of a second at moderate speed. During that time, the valves will be either open or closed depending on the current stroke.
Compression - With both the intake and exhaust valves closed, the pistons upward stroke compresses or squeezes the fuel into the combustion chamber.
Firing - Ignited by the spark, the compressed fuel explodes and forces the piston downward on its power stroke. Intake and exhaust valves are closed. Exhaust valves and seats are exposed momentarily to temperature as high as 3800 degrees F.
Exhaust - The piston moves upward on its exhaust stroke and forces the burned fuel through the open exhaust valve. Intake valve is closed.
The chief reason for loss of power and performance (with gas and oil wasted), is faulty valve action. Carbon, corrosion and wear and misalignment are inevitable products of normal engine operation.
Carbon - Carbon is a by-product of combustion and prevents heat dissipation. Clean metal is a good heat conductor, but carbon insulates and retains the heat. This increases combustion chamber temperature and causes warping and burning. Unburned carbon residue gums valve stems and causes sticking in the guide. Deposits of hard carbon with sharp points projecting become white hot and cause pre-ignition and pinging. Carbon fouls spark plugs, which makes the engine miss and wastes fuel. Valve seats are held open and pitted when held open by carbon particles. Carbon formation cannot be eliminated but is minimized by high quality fuel and valve tune-ups.
Corrosion and Wear - Warping, burning, pitting and out-of-round wear attack the exhaust valve. It is exposed to high temperatures or exhaust gases. Burning and pitting are caused by the valve failing to seat tightly, permitting exhaust blow-by. This condition is often caused by hard carbon particles on the valve seat. It may also be due to weak valve springs, insufficient tappet clearance, warpage, and misalignment. Warpage occurs chiefly in the upper valve stem due to its exposure to intense heat. Out-of-round wear follows when the seat is pounded by a valve whose head is not in line with the stem and guide. Oil and air are sucked past worn intake valve stems and guides into the combustion chamber, causing excessive oil consumption, forming carbon, and diluting carburized fuel.
Misalignment - is a product of wear, warpage, and distortion. Wear perhaps hastened by insufficient or improper lubrication will eventually create sloppy clearances and misalignment. Distortion is caused by unequal tightening of cylinder head bolts. Warpage of the entire engine block is not uncommon, usually due to excessive heat caused by a clogged cooling system. More frequently the valve guide warps because of the variation in temperatures over its length. The lower part of the guide is near combustion heat while the upper is cooled by water jackets. Any wear, warpage or distortion affecting the valve guides, destroys its function as an accurate bearing to keep the valve head concentric with its seat and prevents leakproof sealing.
Valve Spring - Valve springs must be of a uniform length. Place springs on a level surface, see that ends are flat, and use a straight edge to determine irregularity in height. Unequal or cocked valve springs should be replaced. Spring tension that is too weak will allow the valves to flutter. This aggravates wear on valve and seat with possible valve breakage. If the springs are less than 1/16" shorter when compared with a new one, they should be replaced.
Stems and Guides - Check valve stems for wear. Replace valves and guides if worn excessively. Too much clearance in the intake guide admits air and oil into the combustion chamber, upsetting carburetion, increasing oil consumption, and making heavy carbon deposits. Oil passing the intake valve guides can easily be detected because upon inspection, the underside of the intake valves will show a carbon formation. Sloppy exhaust guide clearance causes misalignment and bad seating, resulting in fast valve seat wear.
Sticking Valves - Are caused by carbon, misalignment, and excessive wear and heavily leaded or heavy gum content in the gasoline.
Tappet Clearance - Adjust tappet clearance according to specifications. Correct clearance contributes to quite engine operating and long valve seat wear. Insufficient clearance causes the valve to ride open, resulting in lost compression and burning. Too much clearance retards timing and shortens valve life above seat preventing maximum intake and exhaust. Check tappet guide clearance. Sloppy fit permits tappet to strike valve stem off-center, causing side thrust on valve stem with excessive wear and bad seating.
Timing - Adjust ignition timing with accuracy. Incorrect timing will cancel all benefit of precision reconditioning.
Ignition - With part of every valve job, inspect spark plug electrodes and insulation. Clean plug and adjust specifications. A good spark is necessary for complete combustion. Check ignition system upon which spark depends.