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Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
Insert a rubber wedge between the muffler and
the exhaust port. Tighten the muffler to seal the
exhaust port. Install an adapter, Figure 9-43,
into the spark plug hole. The piston should
be rotated toward the bottom of the stroke to
uncover the tops of the transfer ports. Attach
a pressure-vacuum tester to the adapter. Pres-
surize the engine to 7 psi. The pressure should
hold for about 20 seconds. This verifies there is
no leak with the crankshaft seals or crankcase
gaskets.
Gasoline and Oil Premix Evaluation
Poor fuel quality and fuel contamination are
common causes of problems in two-stroke
engines. These causes are often overlooked
by repair technicians, and the carburetor is
blamed. Too often the carburetor is replaced
and the engine is then tested with the techni-
cian’s known good fuel. The engine runs good,
so the repair technician concludes the original
carburetor was bad and returns the unit to the
customer. When the customer eventually refills
the tank with their own bad fuel, the engine
reverts back to its poor running or nonrunning
condition. It is important for the repair techni-
cian to evaluate the customer’s fuel. The typical
customer may not be aware of reasons why fuel
goes bad. It is important to discuss these factors
with the customer.
Visually Inspect the Customer’s Fuel
Look inside the tank for evidence of debris.
Look at the customer’s fuel for debris, both large
as well as very tiny particles. Eventually, the tiny
particles find their way through the fuel filter
and into the carburetor. They will accumulate
on the pump screen and obstruct fuel delivery
passages in the metering chamber.
Look for water in the fuel. Depending on the
container the fuel is stored in, the water may be
hard to see. The water will be a slightly different
color than the fuel and will form a layer at the
bottom of the fuel. Figure 9-44 shows a small
“puddle” of water in the bottom of the fuel.
The water can come from the hygroscopic
nature of ethanol. Each time a fuel system is
opened during the refueling process, a fresh
supply of humid air enters both the engine fuel
tank as well as the consumer’s storage con-
tainer. As the two-stroke engine runs, the agita-
tion causes the ethanol in the gasoline to absorb
moisture from the air. Simply from the process
of the consumer closing the container and mov-
ing it to another location, the agitated ethanol
absorbs the moisture. Eventually. the ethanol
becomes saturated with moisture and sepa-
rates from the gasoline through a process called
phase separation. The ethanol-plus-water layer
is heavier than gasoline and sinks to the bottom
of the gasoline.
Adapter Rubber plate Rubber wedge
Goodheart-Willcox Publisher
Figure 9-43. The intake and exhaust ports are
sealed off before a crankcase pressure-vacuum
test is performed. The adapter allows pressure
and vacuum to be applied to the crankcase. The
vacuum test can help identify leaking crankshaft
seals. The pressure test can identify leaks in the
crankcase and engine.
Debris
Phase-separated
ethanol and water
Premix
Goodheart-Willcox Publisher
Figure 9-44. The ethanol in the fuel bonds with
moisture and separates from the fuel forming a
phase-separated layer at the bottom of the fuel.