Ignition
Operation Trigger Module (cont'd)
and allows no current
l3
or l4
to
flow.
5.
As the voltage produced in the primary coil
reaches
its
negative peak (point
"b
in
Figure 53),
transistor Tr1 is turned on and allows small current
l3
and large current l4 to
flow.
See Figure 58.
MTI
unit Ignition
coil
Ground
Spark
plug
Figure
58
6.
When transistor Tr1 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 I1 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 Tr1 slightly. This retards the timing and
prevents 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 electrodes 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.
TERMINAL NUT
METAL SHELL
CENTER LEG INSULATOR
ELECTRODE
GROUND ELECTRODE GAP
Figure
59
The other important area is the insulator. The insulator
prevents arcing from taking place in another portion of
the plug, away from the electrodes. Because of the
extremely high voltage present, even a slight crack or
fouling of the head insulator can result in arcing and a
malfunction 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 TC3000, TC4000, and TC5000
is adjustable.
It
is important
to
set
it
to the proper
specification to ensure strong spark and proper timing.
TC3000/TC4000 Air Gap Adjustment Preparation
1.
For convenience, remove the engine from
the
drive
tube as described under Engine Removal from
Drive Tube, page 40.
2.
With the recoil assembly on a hard flat surface, use
an impact wrench to remove the four fan housing
screws. Remove the fan housing.
TC3000/TC4000 Air Gap Adjustment
NOTE: If coil performance is suspect, check the air gap
with a feeler gauge prior
to
loosening
the
two
coil
mounting screws.
It
should be 0.4
to
0.5 mm
(0.016
to
0.020').
1. Loosen the
two
coil mounting screws. Position a
feeler gauge between the coil and the flywheel near
one of the coil mounting screws and tighten.
Repeat this procedure for the other end of the coil.
Air gap adjustment is now complete. See Figure
60.
27
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