Two-Stroke Pressure Testing - Yamaha MM700A Manual

TROUBLESHOOTING
Excessive Vibrations
Excessive vibrations may be caused by loose engine,
suspension or steering mount bolts.
Engine Noises
A change in the sound of the engine is often the first
clue that an engine problem exists. Noises are difficult to
differentiate and even harder to describe. Experience is
needed to accurately diagnose problems using engine
sounds (Figure 26).

TWO-STROKE PRESSURE TESTING

Hard to start and generally poor performing two-stroke
engine may be caused by leaking crankcase compression.
Fuel delivery and ignition systems may be good, and a
compression test may indicate that the engine's upper end
is in good condition but a conventional compression test
does not show a lack of primary (crankcase) compression.
In a two-stroke engine, the crankcase must be alternately
under pressure and vacuum. After the piston closes the in-
take port, further downward movement of the piston
causes the trapped mixture to be pressurized so it can rush
quickly into the cylinder when the scavenging ports are
opened. Upward piston movement lowers the pressure
creates a vacuum) in the crankcase, drawing the air-fuel
mixture in from the carburetor.
NOTE
The operational sequence of a two-stroke
engine is illustrated in Chapter One.
If crankcase seals or cylinder base gaskets leak, the
crankcase cannot hold either pressure or vacuum and
DETONATION
Very rapid combustion
of leakage such as porous or cracked crankcase castings
leaks. The test is simple but requires special equipment. A
typical two-stroke pressure test kit is shown in Figure 27.
Pressure testing requires plugging all engine openings,
then applying air pressure. If the engine does not hold air,
a leak is present that must be located and repaired.
test.
1. Remove the carburetors as described in Chapter Five.
2. Insert a plug tightly in the intake manifold.
3. Remove the exhaust pipes and block off the exhaust
ports, using suitable adapters and fittings.
4. Remove one spark plug and install the pressure gauge
adaptor into the spark plug hole. Connect the pressurizing
lever and gauge to the installed adapter, then squeeze the
lever until the gauge indicates approximately 9 psi.
5. Observe the pressure gauge. If the engine is in good
condition, the pressure should not drop more than 1 1/2 to
2 psi in several minutes. Any pressure loss of 1 psi in one
minute indicates serious sealing problems.
6. Before condemning the engine, first make sure there
are no leaks in the test equipment or sealing plugs. If the
equipment shows no leakage, go over the entire engine
carefully. Large leaks can be heard. Smaller leaks can be
found by going over every possible leakage source with a
Early expansion causes
knock blowby
TDC
proper engine operation is impossible. Any other source
results in the same conditions.
It is possible to test for and isolate engine crankcase
The following procedure describes a typical pressure
NOTE
The labyrinth seal at the center o f the crank-
shaft prevents the cylinders from being
checked individually. When one cylinder is
pressurized, the other cylinder& are also
pressurized. All cylinders must be blocked
before applying pressure during testing.
53
Little expansion after
TDC results in power
loss