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CESSNA
MODEL 172N
SECTION
7
AIRPLANE
&
SYSTEMS DESCRIPTIONS
position. Spring clips will bold the valve open. After draining, use a
suitable tool to s n a p the valve into the extended (closed) position and
remove the drain hose.
IGN ITION-STARTER SYSTEM
Engine ignition i s provided by a n engine-driven dual magneto, and
two s p a r k plugs in each cylinder. The right magneto fires the lower right
and upper left s p a r k plugs, and the left magneto fires the lower left and
upper right s p a r k plugs.'Normal operation i s conducted with both magne-
tos due to the more complete burning of the fuel-air mixture with dual
ignition.
Ignition and starter operation i s controlled by a rotary type switch
located on the left switch and control panel. The switch i s labeled clock-
wise, OFF, R, L, BOTH, and START. The engine should be operatedon both
magnetos (BOTH position) except for magneto checks. The
R
and L
positions are for checking purposes and emergency use only. When the
switch is rotated to the spring-loaded START position, (with the master
switch in the ON position), the starter contactor i s energized and the
starter will crank the engine. When the switch i s released, it will automati-
cally return to the BOTH position.
AIR INDUCTION SYSTEM
The engine a i r induction system receives r a m air through a n intake in
the lower front portion of the engine cowling. The intake i s covered by an
a i r filter which removes dust and other foreign matter from the induction
air. Airflow passing through the filter enters an airbox. After passing
through the airbox, induction a i r enters the inlet in the carburetor which is
under the engine, and i s then ducted to the engine cylinders throughintake
manifold tubes. In the event carburetor ice i s encountered o r the intake
filter becomes blocked, alternate heated air can be obtained from a shroud
around a n exhaust riser through a duct t o a valve, i n the airbox, operated
by the carburetor heat control on theinstrument panel. Heated air from the
shroud i s obtained from a n unfiltered outside source. Use of full carburetor
heat at full throttle will result i n a l o s s of approximately 100 to 225 RPM.
EXHAUST SYSTEM
Exhaust g a s from each cylinder-passes through riser assemblies to a
-,.
muffler and tailpipe. The muffler i s constructed with a shroud around the
outside which forms a heating chamber for cabin heater air.
CARBURETOR AND PRIMING SYSTEM
The engineis equipped with a n up-draft, float-type, fixed jet carburetor
1 July 1979
7-19
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