Spark Plugs - TM-5-4320-258-120053

tor in the primary circuit speeds the collapse of the

(4) Inspect the electrodes for severe burning,

circuit, increasing the secondary voltage and also

cracks, and other damage. If it is necessary to reuse

reducing contact point burning.

the plugs, correct the contact gap to 0.025 inch.

d. The voltage induced in the secondary coil is

(5) Replace any spark plugs whose condition

applied to the spark plug of the required cylinder at the

is doubtful.

exact correct instant by the distributor rotor in the

c. Installation.

magneto. The breaker points open at the required

(1) Make sure all spark plugs are gapped to

instant to assure a hot spark to the spark plug at this

0.025 inch before installation.

time. The spark at the spark plug causes the ignition of

(2) Install the spark plugs, using a socket and

the fuel-air mixture in the cylinder.

torque wrench. Tighten the spark plugs to 35 foot-

e. At starting, an impulse coupling on the magneto

pounds torque.

causes a snapping action of the magneto rotor to

(3) Install the shielded spark plug cables on

produce the rapid rotation necessary to energize the

the spark plugs.

Make sure the leads are not

electrical windings. This retards the rotation of the

interchanged. Refer to figure 4-22 to assure proper

magneto rotor at slow speeds until top-dead-center is

magneto-to-spark plug wiring if spark plug connection is

reached. Spring action then snaps the rotor forward to

doubtful.

produce the required snap.

4-34. Magneto

f. The magneto also contains automatic spark

a. Removal.

advance provisions so that the spark will occur sooner

(1) Disconnect the shielded spark plug cables

during high speed operation. This is necessary for

from the magneto. Disconnect the primary lead to the

smooth engine performance. The automatic spark

magneto.

advance rotor has pawls which are thrown outward as

engine speed increases to change the relationship of the

(2) Remove the lower cap screw (19, fig. 4-

distributor rotor in the magneto.

21) and lock washers (20 and 22), and flat washer (21)

that secure the bottom of the magneto to the engine

4-33. Spark Plugs

front plate. Remove the nut (16) and lock washer (17)

a. Removal.

from the bolt (33, fig. 4-11) that secures the ground

(1) Use compressed air to blow dirt away

strap (18, fig. 4-21), the top of the magneto, and the top

from the spark plugs before removing them. This will

of the engine speed governor to the gear cover; remove

prevent the dirt from entering the cylinder as the spark

the magneto (23), gasket (24), and spacer (25).

plug is removed.

b. Replacing and Adjusting Breaker Point

(2) Disconnect the shielded spark plug cables

Assembly.

from the spark plugs (13, fig. 4-21) and remove the

(1) Remove the screws (1 and 2, fig. 4-23)

spark plugs, using a socket wrench.

and lock washers (3) that secure the distributor housing

b. Cleaning and Inspection.

(7) to the magneto frame. Pull the distributor housing

Note

away from the frame far enough to disconnect the

It is normally more economical to

capacitor lead from the terminal on the breaker point

replace the spark plugs than to clean

assembly. Remove the distributor housing. Remove

and regap them. It also provides

and discard the distributor housing gasket (6).

greater assurance that there are no

(2) Use a spanner wrench to remove the

hidden cracks in the ceramic

capacitor (4) from the distributor housing and remove

insulators which could cause engine

the preformed packing (5). Discard both parts.

misfiring at high engine speeds.

(3) Remove the nut that secures the coil lead

Cleaning and inspection instructions

to the breaker point assembly (14) and disconnect the

are given here in case it is necessary

lead.

to reuse the removed plugs.

(4) Remove the two screws (10 and 11), lock

(1) Wipe the exterior of the spark plugs with a

washers (12), and flat washer (13) that secure the

cloth dampened with cleaning solvent (fed. spec. P-D-

breaker point assembly to the rotor bearing plate (28);

680); dry thoroughly.

remove the breaker point assembly.

(2) Check the ceramic insulator for cracks,

porosity, and other damage.

(3) If the electrodes of the spark plugs are not

too badly burned, remove the scale and other deposits

from them. This can be done by a sandblast-type

cleaner or with a contact file.

4-24