Engine Tune-Up - Yamaha MM700A Manual

74
Electrical Connectors
Inspect the high tension leads to the spark plugs for
cracks and breaks in the insulation and replace the leads if
they arc not perfect. Breaks in the insulation allow the
spark to arc to ground and impair engine performance.
Check primary ignition wiring and lighting wires for
damaged insulation. Usually minor damage can be re-
paired by wrapping the damaged area with electrical insu-
lating tape. If insulation damage is extensive, replace the
damaged wires.
Abnormal Engine Noise
Never lean into the snowmobile's engine
compartment while wearing a scarf or other
loose clothing when the engine is running or
when anyone is starting the engine. ifthe
scarf or clothing should catch in the drive
belt or clutch, severe injury could occur.
Make sure the belt guard is in place.
Open the hood, then start the engine and listen for ab-
normal noises. Often the first indication of trouble is a
change in sound. An unusual rattle might indicate a loose
fastener that can be easily repaired or the first warning
sign of excessive engine damage. With familiarity of the
machine and practice, the home mechanic will be able to
identify most new sounds. Periodic inspection for abnor-
mal engine noises can prevent engine failure later on.

ENGINE TUNE-UP

The following paragraphs discuss the different parts of
a tune-up. Perform them in the order given. Have the new
parts on hand before beginning.
Cylinder Head Nuts
The engine must be at room temperature for this proce-
dure.
Open the hood.
2. Tighten each cylinder head retaining nut in the se-
quence shown in Figure 34 to 23 N*m (17 ft.-lb.).
Cylinder Compression
A cylinder cranking compression check is one of the
quickest ways to check the condition of the piston rings
and head gasket. It is a good idea to check compression at
each tune-up, record the compression of each cylinder,
then compare the current compression with test results
from earlier tune-ups. The first step is to record the mea-
sured compression of each cylinder and the date, so it can
be compared with tests recorded at the next tune-up. A
gradual change may indicate normal wear or may help in
spotting a developing problem.
1. Elevate and support the machine so the track is com-
pletely off the ground and free to rotate. Run the engine
until it warms to normal operating temperature, then turn
the engine off.
Do not stand behind or in front of the ma-
chine when the engine is running, and keep
hands, feet and clothing (may from the track
when it is turning.
CAUTION
To prevent expensive engine damage, use
compressed air to blow away any dirt that
has accumulated next to the spark plug
base. The dirt could fall into the cylinder
when the plug is removed, causing serious
engine damage.
2. Remove all the spark plugs, then attach the plugs to the
caps at the ends of the spark plug leads and ground both
plugs to the cylinder head.
CAUTION
I f the plugs are not grounded during the
compression test, the
damaged.
3. Screw a compression gauge into one spark plug hole,
or if there is a press-in type gauge on hand, hold it firmly
in position.
4. Check that the emergency cut-out switch is in the OFF
position.
5. Hold the throttle wide open and crank the engine sev-
eral revolutions until the gauge gives its highest reading.
Record the reading, indicating the cylinder (MAG end or
PTO end). Remove the pressure tester and relieve the
pressure valve.
Repeat Steps 3-5, for the other cylinders.
6.
7. There should be no more than a 10 percent difference
in compression between cylinders.
8. If the compression is very low, a ring is probably bro-
ken or there is a hole in the piston.
9. Reinstall the spark plugs and reconnect the spark plug
wires.
CHAPTER THREE
ignition could be