John Deere stx38 - Manual (Page 66)

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4/23/97
REPAIR ENGINE
ANALYZE VALVES
Lead deposits on the intake valve are caused by
exhaust gas leakage past the valve when using leaded
gasoline. Grind intake valve and reface valve seat to
correct this condition.
Use unleaded fuel to prevent lead deposits.
Valve stem corrosion is caused by moisture in the
engine which occurs during hot engine cool-down
periods or during storage.
Fogging of combustion chamber with oil before storage
helps prevent corrosion. Replace badly corroded
valves.
Operating at high temperatures for long periods of time
can cause exhaust valve burning. Burned valve will
show dark discoloration into the area protected by the
valve guide. Another indication is distortion of the
margin (A) and face angle (B). The valve seat may also
show erosion.
An overheated engine can also cause valve burning.
Check for clogged engine cooling fins. Do not run
engine with blower housing removed. Also check for
worn valve guides, springs or hydraulic lifters, lean
fuel-air mixture, or incorrect spark plug.
Use of old or stale gasoline can cause valves to stick.
Gummy deposits (C) can build up on valve and can
also gum carburetor, requiring cleaning.
Always use fresh gasoline of 87 octane rating or
higher. Drain fuel tank, lines, and carburetor before
storage.
M29934
M55630
M30024
A
B
M29936
C