Engine Valves - Some Helpful Information
The complete cycle of intake, compression, firing and exhaust takes place in less
than 1/25 of a second at moderate speed.
Intake - On the intake stroke the piston moves doward, sucking in
carburized fuel through the open intake valve. Exhaust valve is closed.
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
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
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 entirely eliminated but is minimized by high quality fuel and valve
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
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
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.
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