Air Gap Adjustment - Toro TC 1000 Service Manual

Ignition Operation Trigger Module (cont'd)
5. As the voltage produced in the primary coil
reaches its negative peak (point "b" in Figure
50). transistor T r l is turned on and allows
small current l3 and large current l4 to flow.
See Fig. 55.
MTI
unit
Ignition coil
Ground
Spark plug
Figure 55
6. When transistor T r l turns on, nearly all of the
current flow through R4 and Tr2 is diverted
through path l4 since it is the path of least
resistance. This drop in current l1 results in
transistor Tr2 turning off.
7. When Tr2 turns off, current l2 drops rapidly
and causes the magnetic field surrounding
the primary coil to rapidly collapse. This in
turn causes a voltage surge in the secondary
which is sufficient to produce a spark across
the spark plug.
Another task the trigger module performs is to
limit the maximum revolutions per minute that
the engine will attain. It does this by means of the
ITDC (ignition timing delay circuit) which can
also be seen in Figure 55.
This circuit senses the engine speed, and, as
it approaches 10,000 rpm, it delays the turning
on of transistor T r l slightly. This retards the
timing and protects the engine from further
acceleration.
Ignition Operation Spark Plug
The spark plug is used to ignite the air/fuel
mixture by producing a spark just before the
piston reaches top dead center. A spark plug is
typically constructed as shown in Figure 56.
There are two critical areas important to proper
spark plug function. The first is that the elect-
rodes are properly gapped and are clean. This
ensures that a strong spark will be present and
that it occurs at the proper time. Excessive gap or
fouling can delay firing enough to cause a loss of
power or stalling.
The other important area is the insulator. The
insulator prevents arcing from taking place in
25
TERMINAL NUT
METAL SHELL
ELECTRODE
HEAD INSULATOR
GROUND ELECTRODE
Figure 56
another portion of the plug, away from the
electrodes. Because of the extremely high volt-
age present, even a slight crack or fouling of the
head insulator can result in arcing and a mal-
function of the plug.
AIR GAP ADJUSTMENT
The space between the coil and the flywheel
magnets is called the "air gap". Because the coil
mounting holes are oversized, air gap on the TC
1000 is adjustable. It is important to set it to the
proper specification to ensure strong spark and
proper timing.
Air Gap Adjustment Preparation
1.
For convenience, remove the engine from
the drive tube as described under Engine
Removal from Drive Tube, page 34.
2. Remove the three Phillips screws retaining
the plastic fan housing cover to the fan
housing. Remove the cover.
3. With the recoil assembly on a hard flat
surface, use an impact wrench to
remove
the
four fan housing screws. Remove the fan
housing.