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restricts the fl ow of the fuel/air mixture (fuel/oil/air mixture
for two stroke) and reduces power. If enough ice builds up,
the engine may cease to operate. Carburetor ice is most likely
to occur when temperatures are below 70 °F (21 °C) and
the relative humidity is above 80 percent. However, due to
the sudden cooling that takes place in the carburetor, icing
can occur even with temperatures as high as 100 °F (38 °C)
and humidity as low as 50 percent. This temperature drop
can be as much as 60 to 70 °F. Therefore, at an outside air
temperature of 100 °F, a temperature drop of 70 °F results in
an air temperature in the carburetor of 30 °F. [Figure 4-14]
The fi rst indication of carburetor icing is a decrease in engine
rpm, which may be followed by engine roughness. Although
carburetor ice can occur during any phase of fl ight, it is
particularly dangerous when using reduced power during a
descent. Under certain conditions, carburetor ice could build
unnoticed until trying to add power. To combat the effects of
carburetor ice, some engines have a carburetor heat option.
Some of the newer four-stroke engines have carburetor heat
turned on all the time to combat icing. Two-stroke engines
are typically less susceptible to icing but specifi c installations
Two-Stroke Carburetor Jetting for Proper Mixture
Carburetors are normally set at sea level pressure with
the jets and settings determined by the manufacturer.
[Figure 4-13] However, as altitude increases, the density
of air entering the carburetor decreases, while the density of
the fuel remains the same. This creates a progressively richer
mixture, same fuel but less air, which can result in engine
roughness and an appreciable loss of power. The roughness
is usually due to spark plug fouling from excessive carbon
buildup on the plugs. Carbon buildup occurs because the
excessively rich mixture lowers the temperature inside the
cylinder, inhibiting complete combustion of the fuel.
4-11
Relative humidity
Outside air temperature
100%
50%
80%
60%
70%
90%
20 °F/–7 ° C 32 °F/0 °C 70 °F/21 °C 100 °F/38 °C
High carburetor
icing potential
Carburetor icing possible
Figure 4-14. Although carburetor ice is most likely to form when
temperature and humidity are in ranges indicated by this chart,
carburetor ice is also possible under conditions not depicted.
Fuel tank
Fuel lines to each cylinder
Fuel manifold valve
Electric fuel pump
Fuel filter
Throttle control
Fuel/air control unit
Fuel
pressure
regulator
Figure 4-15. Fuel injection system.
dictate how susceptible the carburetor is to icing. Consult
the aircraft POH for the probability of carburetor ice for the
specifi c installation and for carburetor ice procedures.
Fuel Injection Induction Systems
In a fuel injection system, the fuel is injected either directly
into the cylinders or just ahead of the intake valve. A fuel
injection system usually incorporates these basic components:
engine-driven fuel pump, fuel/air control unit, fuel manifold
(fuel distributor), discharge nozzles, auxiliary fuel pump, and
fuel pressure/fl ow indicators. [Figure 4-15]
The engine-driven fuel pump provides fuel under pressure
from the fuel tank to the fuel/air control unit. This control
unit, which essentially replaces the carburetor, meters the fuel
and sends it to the fuel manifold valve at a rate controlled
by the throttle. After reaching the fuel manifold valve, the
fuel is distributed to the individual fuel discharge nozzles.
The discharge nozzles, which are located in each cylinder
head, inject the fuel/air mixture at the precise time for each
cylinder directly into each cylinder intake port.
Some of the advantages of fuel injection are:
• No carburetor icing.
• Better fuel fl ow.
• Faster throttle response.
• Precise control of mixture.
• Better fuel distribution.
• Easier cold weather starts.
Disadvantages include:
• Diffi culty in starting a hot engine.
• Vapor locks during ground operations on hot days.
• Problems associated with restarting an engine that
quits because of fuel starvation.
4-12
Figure 4-16. Keyed ignition system with integral starter.
Ignition System
The typical ignition system on WSC aircraft provides the
spark that ignites the fuel/air mixture in the cylinders and
is made up of magneto/generators, control boxes, spark
plugs, high-voltage leads, and the ignition switch. For most
　
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