Cessna 150J - Magneto Check; Maintenance; Spark Plugs; Engine Controls

in normal direction of rotation until timing light in-
dicates position at which contact points break. The
contact points should break at the advanced firing
pOSition of number one cylinder. Loosen mounting
nuts slightly and rotate magneto case to make con-
tact points break at the correct position. Tighten
mounting nuts.
i. After tightening magneto mounting nuts, recheck
timing. Make sure both magnetos are set to fire at
the same time. Remove timing equipment and con-
nect spark plug leads and ignition switch leads.
NOTE
Beginning with the number one outlet, the
magneto fires at each successive outlet in
a counterclockwise direction, looking at
the outlets. Connect number one magneto
outlet to number one cylinder spark plug
lead, number two outlet to the next cylinder
to fire, etc. Engine firing order is listed
in paragraph 11-8.
11-46A. MAGNETO CHECK. Advanced timing set-
tings in some cases, is the result of the erroneous
practice of bumping magnetos up in timing in order
to reduce RPM drop on Single ignition. NEVER AD-
VANCE TIMING
BEYOND SPECIFICATIONS IN OR-
DER TO REDUCE RPM DROP. Too much impor-
tance is being attached to RPM drop in single igni-
tion. RPM drop on single ignition is a natural char-
acteristic of dual ignition design. The purpose of
the following magneto check is to determine that all
cylinders are firing. If all cylinders are not firing,
the engine will run extremely rough and cause for
investigation will be quite apparent. The amount of
RPM drop is not necessarily Significant and will be
influenced by ambient air temperature, humidity,
airport altitude, etc. In fact, absence of RPM drop
should be cause for suspicion that the magneto timing
has been bumped up and is set in advance of the set-
ting specified. Magneto checks should be performed
on a comparative basis between individual right and
left magneto performance.
a. Start and run engine until the oil and cylinder
head temperatures are in the normal operating ranges.
b. Advance engine speed to 1700 rpm.
c. Turn the ignition switch to the "R" position and
note the rpm drop, then return the switch to the
"BOTH" position to clear the opposite set of plugs.
d. Turn the switch to the "L" position and note the
rpm drop, then return the switch to the "BOTH"
pOSition.
e. The rpm drop should not exceed 150 rpm on
either magneto or show greater than 75 rpm differ-
ential between magnetos. A smooth rpm drop-off
past normal is usually a sign of a too lean or too
rich mixture. A sharp rpm drop-off past normal
is usually a sign of a fouled plug, a defective harness
lead or out of time. If there is doubt concerning op-
eration of the ignition system, rpm checks at a leaner
mixture setting or at higher engine speeds will usually
confirm whether a deficiency exists.
NOTE
An absence of rpm drop may be an indica-
tion of faulty grounding of one side of the
ignition system, a disconnected ground
lead at magneto or possibly the magneto
timing is set too far in advance.
11-47. MAINTENANCE. Magneto-to-engine timing
should be checked at the first 50 hours, first 100
hours, and thereafter at each 200 hours. If timing
to the engine is not within plus zero degrees and
minus two degrees, the magneto should be timed to
the engine.
NOTE
If ignition trouble should develop, spark plugs
and ignition wiring should be checked first.
If the trouble appears definitely to be associ-
ated with a magneto, the following may be used
to help disclose the source of trouble.
a. Remove high-tension outlet plate and check dis-
tributor block for moisture.
b. If any moisture is evident, lightly wipe with a
soft, dry, lint-free cloth. Reinstall outlet plate.
NOTE
Since these magnetos MUST NOT BE DISASSEM-
BLED, new
magnetos should be installed if the
moisture check does not remedy the trouble.
11-48. SPARK PLUGS. Two 18-mm spark plugs are
installed in each cylinder and screw into heli-coil
type thread inserts. The spark plugs are shielded to
prevent spark plug noise in the radio and the spark
plugs have an internal resistor to provide longer
terminal life. Spark plug life will vary with operat-
ing conditions. A spark plug that is kept clean and
properly gapped will give better and longer service
than one that is allowed to collect lead deposits and
is improperly gapped. The correct spark plug and
gap setting is given in paragraph 11-8.
NOTE
At each 100-hour inspection, remove, clean,
inspect, and regap all spark plugs. Install
lower spark plugs in upper portion of cylinder
and install upper spark plugs in lower portion
of cylinders at each 100-hour inspection.
Since deterioration of lower spark plugs is
usually more rapid than that of the upper spark
plugs, rotating them helps prolong spark plug
life.
11-49. ENGINE CONTROLS.
11-50. Engine controls of the push-pull type include
the throttle, mixture, and carburetor heat controls.
The engine controls are equipped with position-lock-
ing devices which prevent vibration-induced "creep-
ing" of the controls.
11-21