Linhai HY740STL 2018 Service Manual page 125

CHAPTER 3 ENGINE
DO: Spark plug wires(secondary leads) & primary
wiring:
- must not contact sharp surface
- must not be under tension between fixed points
- must be clear of moving parts (belts, fan, etc...)
- must be protected from or kept at least 125 mm
away from radiant heat source exceeding 400 F.
- must be protected from environmental damage
(dirt, splash, oils, fluids, etc....)
- must be retained, secured or insulated to prevent
pinching, mis-routing, rattles, and squeaks
DO: Not all fasteners are designed for repeat use.
Beware of fastener specifications. All harnesses
should be supported within 6" of a mating
connection.
DO: For removing spark plugs follow the following
steps:
1- Grasp the spark plug boot and gently rotate
90 ; and then pull the spark plug boot and
cable away from the spark plug
2- Before removing spark plug, brush or air blast
dirt away from the well areas
3- Use correct size deep socket wrench to loosen
each spark plug one or two turns
DO: Cleaning a spark plug could be done as follow:
1- wipe all spark plug surfaces clean....remove
oil, water, dirt and moist residues.
2- If the firing end of spark plug has oily or wet
deposit, brush the spark plug in an approved,
non-flammable and non-toxic solvent. Then
dry the spark plug thoroughly with compressed
air
3- Use a propane torch to dry wet-fuel fouled
plugs. Allow the torch flame to enter up the center
electrode insulator. Allow plug to cool down
4- If the spark plug threads have carbon & scale
deposits, clean with wire brush, taking care not
to injure the electrode or the insulator tip
DO: Regap spark plugs to the exact measurement
specified by the engine manufacturer to keep the
best fuel economy and proper engine performance
- Use round wire-type gauge for an accurate
measure of gap on all used spark plugs
- when gapping a spark plug only the side
electrode
is moved. The center electrode must not be moved
DO: When replacing spark plugs with new ones,
always use equivalent plugs with same heat range,
thread, size, etc....
DO: For installing spark plugs follow the
following
steps:
1- make sure the cylinder head threads and spark
plug threads are clean. Make sure the spark
plug thread is free of dings and burrs. If
necessary, use a thread chaser and seat
cleaning tool.
2- Make sure the spark plug gasket seat is clean,
then thread the gasket to fit flush against the
gasket seat. Tapered seat plugs do not require
failure condition
- Spark plug wires carry very high voltage (30,000
volt). If the secondary lead loses its dielectric
characteristics thru being nicked, cut , chaffed, then
an arc thru to a near by ground could take place. This
kind of condition could lead to misfire, no start, or
premature failure of ignition system.
Adequate retention force might not be achieved if the
fastener is not designed to be reused. Mating
connections are not designed to support the weight
of the harness assembly
To remove spark plugs from Aluminum heads, allow
the engine to cool. The heat of the engine, in
combination with a spark plug that is still hot, may
cause the spark plug threads to strip the cylinder
head
upon removal
Use goggles to protect eyes from dirt when applying
compressed air to spark plug wells
- Cleaning a spark plug will reduce the voltage
required for an electrical arc(spark) across the
electrodes
- Cleaning & re-gapping will not restore a used
spark plug to a new condition. It may be more
economical and efficient to replace used spark
plugs with new plugs instead of cleaning.
- Sooted plugs should be replaced
- Do not cool by using water or any liquid
- Clean threads permit easier installation and
proper seating which will maximize transfer heat
away from the plug
- Too wide a gap could cause the plug to
misfire(higher required
ignition voltage).
- Too narrow of a gap could affect idle stability
- A flat gauge can't accurately measure the spark
plug on used plugs
- Higher heat range plug(hotter plug) could lead to
pre-ignition & possible piston damage
- Lower heat range (colder plug) could lead to cold
fouling & emission problem
- If the thread is damage, it prevents a good heat
transform from the shell to the cylinder head
- Do not use any type of anti-seize compound on
spark plug threads. Doing this will decrease the
amount of friction between the threads. The result
of the lowered friction is that when the spark plug
is torqued to the proper specification, the spark
plug is turned too far into the cylinder head. This
increases the likelihood of pulling or stripping the
threads in the cylinder head
- Over-tightening of a spark plug can cause
M750L SERVICE MANUAL 18.0
CHAPTER 3 ENGINE PAGE 3- 94