Section VIII. MAINTENANCE OF IGNITION SYSTEM

Section VIII. MAINTENANCE OF IGNITION SYSTEM

4-14. Description

a. The ignition system consists of a magneto, its

breaker points, and the spark plug which uses a

shielded spark plug cable. The ignition system

supplies a spark at the proper instant necessary to

ignite the fuel-air mixture in the cylinder at the

height of the compression stroke.

b. The magneto consists of an electrical coil

mounted on a laminated steel frame, positioned

within the flywheel. Permanent magnets are case

into the flywheel. As the magnets rotate past the

coil, they induce an electrical current in the elec-

trical coil. The electrical coil has a primary winding

and a secondary winding. The electrical current in

the primary winding induces a high-voltage current

in the secondary winding. The voltage peak is

reached as the breaker points are opened to break

the primary circuit, causing a rapid decay of the

magnetic field. A capacitor in the circuit speeds the

collapse of the magnetic field to cause a hotter

spark at the spark plug. The capacitor also

minimizes burning of the contact points.

c. The breaker point set is opened by a cam on

the camshaft through a breaker rod which rides

against a lobe on the camshaft. The timing of the

point opening is critical and is done with the aid of

timing marks on the engine flywheel. The timing

setting can be done either by static adjustment or

by use of a timing light.

d. The spark plug is a radio-shielded type,

designed to minimize emission of radio interference

Spark plug

resulting from the arcing of the plug. The spark

Gasket

plug cable is also shielded to prevent emission of

Machine screw

radio frequency interference.

Spark plug cable

Magneto coil

4-15. Spark Plug and Cable

Machine screw

a. Removal.

Machine screw

(1) Disconnect the spark plug cable (4, fig. 4-

Capacitor

3) from the spark plug (1); remove the spark plug

Machine screw

10.

Lock washer

and gasket (2) from the cylinder head.

11.

Magneto frame

(2) To remove the spark plug cable (4), it is

necessary to remove the blower housing (para 4-

Figure 4-3. Magneto, spark plug, and cable, exploded view.

12), and the starter pulley and flywheel (para 4-

13). Disconnect the cable shielding and pull the

end of the cable from the cable socket in the

magneto coil (5).

4-7