Kohler Courage SV470 Service Manual page 88

Section 9
Inspection and Reconditioning
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
season. 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.
9.8
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