Jacobsen J-125AS Service Manual page 8

SECTION I - SERVICE AND O P E R A T I O N - - - - - - - - - - - - - -
CHOKE LEVER
Figure 1. Jacobsen Engine Cross Section
Both ports are covered by the piston except for a
short time at the extreme end of the stroke near the
crankshaft.
When the piston has completed the compression of
the fuel mixture at the end of the stroke, away from
the crankshaft, the spark plug ignites the charge.
As it expands, the burning gases drive the piston
away to produce the power stroke on the crankshaft.
This two-stroke cycle is repeated each time the
piston reaches the end of the stroke to uncover the
ports.
The carburetor provides a combustible mixture of
fuel and air to the cylinder for ignition. Fuel is
metered to a stream of air flowing through a venturi
into the cylinder. The proportion of fuel to air is
controlled by a needle valve. The air flow atomizes
the fuel as it is drawn into the engine. Proportion
of fuel to air is important for efficient burning of
fuel. An excess of fuel (rich) produces partial com-
bustion due to insufficient air and results in sluggish
operation of the engine. An excess of air (lean) pro-
duces slow burning of fuel with a resultant loss of
power.
A float chamber maintains a steady supply of fuel
for the fuel-air mixture. Filling of the chamber
raises the float to shut off the flow of fuel. As fuel
is used , the float settles and opens the supply line
for additional fuel.
A flywheel type magneto (figure 2) , a high tension
wire and a spark plug complete the ignition system.
The magneto, a stationary self-contained generating
unit , includes the armature plate assembly (lamina-
tions and ignition coil) , condenser and breaker point
6
assembly. The permanent magnet built into the fly-
wheel revolves around the stationary pole pieces in
the armature plate assembly. As the ends of the
magnet pass by the pole pieces, an alternating mag-
netic flux is established through the ignition coil. A
current is generated in the primary circuit during
the period that the breaker points are closed. A cam
located on the crankshaft opens the points when the
primary current is at a maximum. Interruption of
the current breaks the magnetic field around the coil.
The collapse of the magnetic field induces a high
voltage through the secondary winding of the coil ,
which is then carried through the high tension wire
to the spark plug. The current then jumps the spark
plug gap to ignite the fuel charge in the cylinder.
Each revolution of the crankshaft repeats the cycle
of ignition. The condenser, connected across the
contacts , absorbs the current which tends to contin-
ue after breaking contact. This hastens the collapse
of the magnetic field and reduces pitting of breaker
points by absorbing the sparking across the points.
The governor assembly is a simple unit consisting
of a governor vane located adjacent to the flywheel
and is connected by linkage to the carburetor throttle
lever. The air current produced by the flywheel acts
upon the governor vane. A governor spring has an
opposing effect on the vane. An equilibrium between
air force on the vane and the governor spring force
is created in this way , maintaining the selected
speed.
If
the speed of the engine drops due to a
sudden load,the air current produced by the flywheel
is reduced. Since the force exerted by the governor
spring is greater, it reacts on the throttle lever to
open the throttle and provide more fuel and air to the
engine. The engine speed increases until the equi-
librium of the air force and the governor spring is
at the proper balance for the new load condition.