Kohler CH18-CH26 Service Manual page 203

Excessive Combustion Temperatures: The white
deposits seen here indicate very high combustion
temperatures, usually due to a lean fuel mixture.
Gum: Gum deposits usually result from using stale
gasoline. Gum is a prevalent cause of valve sticking.
The cure is to ream the valve guides and clean or
replace the valves, depending on their condition.
Inspection and Reconditioning
Stem Corrosion: Moisture in fuel or from
condensation are the most common causes of valve
stem corrosion. Condensation occurs from improper
preservation during storage and when engine is
repeatedly stopped before it has a chance to reach
normal operating temperatures. Replace corroded
valves.
Overheating: An exhaust valve subject to
overheating will have a dark discoloration in the area
above the valve guide. Worn guides and faulty valve
springs may cause this condition. Also check for
clogged air intake, and blocked fins when this
condition is noted.
Section 10
10
10.7