Cessna Skyhawk 172N Pilot Operating Handbook page 125

CESSNA
SECTION 7
MODEL 172N
AIRPLANE
&
SYSTEMS DESCRIPTIONS
and upper left spark plugs, and the left magneto fires the lower left and
upper right spark pI ugs. Normal operation is conducted with both magne ­
tos due to the more complete burning of the fuel-air mixture with dual
ignition.
Ignition and starter operation is controlled by a rotary type switch
located on the left switch and control panel. The switch is labeled clock­
wise, OFF, R, L,BOTH, and START. The engine should be operated on both
magnetos (BOTH position) except for magneto checks. The Rand 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 is energized and the
starter will crank the engine. When the switch is released, it will automati ­
cally return to the BOTH position.
AIR INDUCTION SYSTEM
The engine air induction system receives ram air through an intake in
the lower front portion of the engine cowling. The intake is covered by an
air filter which removes dust and other foreign matter from the induction
air. Airflow passing through the filter enters an airbox. After passillg
through the airbox, induction air enters the inlet in the carburetor which is
under the engine, and is then ducted tothe engine cylinders through intake
manifold tubes. In the event carburetor ice is encountered or the intake
filter becomes blocked. alternate heated air can be obtained from a shroud
around an exhaust riser through a duct to a valve, in the airbox, operated
by the carburetor heat control on the instrument panel. Heated air from the
shroud is obtained from an unfiltered outside source. Use of full carburetor
heat at full throttle will result in a loss of approximately 100 to 225 RPM.
EXHAUST SYSTEM
Exhaust gas from each cylinder passes through riser assemblies to a
muffler and tailpipe. The muffler is constructed with a shroud around the
outside which forms a heating chamber for cabin heater air.
CARBURETOR AND PRIMING SYSTEM
The engine is equipped with an up-draft. float-type. fixed jet carburetor
'"",-" mounted on the bottom of the engine. The carburetor is equipped with an
enclosed accelerator pump, an idle cut-off mechanism, and a manual
mixture control. Fuel is delivered to the carburetor by gravity flow from
the fuel system. In the carburetor, fuel is atomized, proportionally mixed
with intake air, and delivered to the cylinders through intake manifold
1 July 1978
7-19