Cessna 150L - Magneto Removal; Internal Timing; Replacement Interval; Magneto Installation and Timing to Engine

11-43. MAGNETO REMOVAL. Remove high-tension
outlet plate, disconnect primary lead, and remove
nuts and washers securing magneto to the engine.
Note the approximate angular position at which the
magneto is installed, then remove the magneto.
11-44. INTERNAL TIMING. Internal timing is
accomplished during manufacture of the magneto.
Since these magnetos are NOT TO BE DISASSEM-
BLED, there is no internal timing involved.
11-45. REPLACEMENT INTERVAL. These magnetos
cannot be overhauled in the field. The coil, capacitor,
and breaker assembly are non-replacable. As a good
maintenance practice, and to have the benefit of good
ignition at all times, it is recommended that the mag-
netos be removed at 900 hours of magneto time, and
install new exchange magnetos.
11-46. MAGNETO INSTALLATION AND TIMING TO
ENGINE. The magneto must be installed with its
timing marks correctly aligned, with number one
cylinder on its compression stroke, and with the
number one piston at its advanced firing position.
Refer to paragraph 11-8 for the advanced firing posi-
tion of number one piston. To locate the compres-
sion stroke of the number one cylinder, remove the
lower spark plug from number two, three, and four
cylinder and remove the upper spark plug from num-
ber one cylinder. Place the thumb of one hand over
the spark plug hole of number one cylinder and rotate
crankshaft in the direction of normal rotation until
the compression stroke is indicated by positive pres-
sure inside the cylinder lifting the thumb off the
spark plug hole. After the compression stroke is
obtained, locate number one piston at its advanced
firing pOSition. Locating the advanced firing posi-
tion of number one piston may be obtained by rotat-
ing the crankshaft opposite to its normal direction
of rotation until it is approximately 30 degrees before
top center (BTC) on the compression stroke of num-
ber one cylinder. Then, rotate crankshaft in normal
direction of rotation to align the correct timing mark
on the outer surface of the crankshaft flange, with
the parting surface of the crankcase below the crank-
shaft as the reference point, making sure the final
motion of the crankshaft is in the direction of normal
rotation.
NOTE
Various other methods and equipment may be
used to arrive at the number one piston ad-
vanced firing position. One such method is:
An accurate top center indicator which screws
into a spark plug mounting hole, and a pendu-
lum pointer mounted on a 360-degree timing
disc may be used. The timing disc should be
adapted to fit over the end of the propeller
spinner in such a manner that it may be ro-
tated as necessary.
In all cases, it must be definitely determined
that No. 1 cylinder is at the correct firing
position, on its compression stroke, when the
crankshaft is rotated in its normal direction
of rotation.
After the engine has been placed to the correct fir-
ing position, install and time magneto to the engine
in the following manner.
a. Remove the timing (vent) plug from the bottom
of the magneto. The vent plug in the top of the mag-
neto need not be removed.
b. Rotate magneto shaft until timing mark on rotor
is visible through the vent plug holeo Impulse cou-
pling pawls must be depressed to turn magneto shaft
in normal direction of rotation.
c. Establish that the magneto is at number one fir-
ing position. It is possible for the timing mark to be
visible while firing position is 180 degrees from
number one firing position.
NOTE
It is necessary to "spark" the magneto to es-
tablish the correct firing position. The outlet
plate with spark plug leads must be installed
on the magneto. Hold number one spark plug
lead (see figure 11-6) close to magneto case,
or ground the magneto and hold the number one
spark plug lead close to a good ground. Ro-
tate impulse coupling in normal direction of
rotation until a spark occurs at this lead.
(Impulse coupling pawls must be depressed to
turn magneto shaft in normal direction of ro-
tation.) Turn impulse coupling backward a
few
degrees, until timing marks on rotor is
centered in vent plug hole and install timing
pin (or 0.093 inch 6-penny nail) through hole
in bottom of magneto next to the flange and
into the mating hole in the rotor shaft. This
locks the magneto approximately in firing
pOSition while installing it on the engine.
d. Keep timing mark centered in vent plug hole dur-
ing installation.
e. Be sure magneto gaskets are in place and that
engine is in correct firing pOSition, then install
magneto (s) at the angle noted during removal, tighten
mounting nuts finger tight.
Remove timing pin from magneto, if installed.
Be sure to remove this pin before rotating
crankshaft.
f. Connect one lead of timing light to the capacitor
terminal at the rear of the magneto and ground lead
of timing light to a good ground.
g. Rotate propeller opposite to normal direction of
rotation a few degrees (approximately 5 degrees) to
close magneto contact points.
NOTE
Do not rotate propeller back far enough to en-
gage impulse coupling, or propeller will have
to be rotated in normal direction of rotation
until impulse coupling releases, then backed
up a few degrees before the firing position.
h. Slowly advance propeller (tap forward with
minute movements as firing position is approached)
11-19