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CHAPTER 9 Two-Stroke Engine Troubleshooting and Evaluation
Copyright Goodheart-Willcox Co., Inc.
step-by-step procedures for testing components
and troubleshooting the ignition system.
Fuel Delivery System
The fuel that was in the equipment when it was
brought in for service needs to be evaluated.
Make a note of debris in the fuel and in the tank.
Look to the bottom of the fuel in the container
to see evidence of water in the fuel. If the engine
had no fuel when it came in, smell the inside of
the tank to determine if fuel had become stale.
If there is a stale fuel odor, the carburetor has
probably been affected. The following areas of
the fuel delivery system need to be evaluated:
• Fuel tank.
• Carburetor.
• Impulse circuit.
• Gasoline tests.
Fuel Tank Evaluation
“Does the engine’s fuel storage system have
a leak?” A two-stroke engine fuel system is a ”
sealed system and should not leak in any posi-
tion. There should be no leaks, even when the
fuel tank pressure increases during normal
engine operation.
At the carburetor, disconnect the fuel return
line and attach a pressure tester. Make sure the
cap is installed on the tank. See Figure 9-31.
Clamp the vent and return lines closed. Pump
1.5 psi pressure into the fuel tank. The pressure
gauge should hold steady. If the pressure leaks
down, spray a solution of soapy water on the
various parts of the system to check for leaks.
Leaks can occur at the tank lid gasket, a crack
in the tank, the grommet, a fuel line, or carbu-
retor. Bubbles will form in the soapy water film
anywhere there is a leak.
If the fuel tank pressure holds, release the
clamp on the tank vent. The tank vent should
continue to hold the 1.5 psi pressure. If the pres-
sure leaks down when the clamp on the vent line
is removed, the vent is leaking and needs to be
replaced.
Remove the vent from the vent line and
attach a pressure/vacuum tester to the vent.
See Figure 9-32. Pressurize the vent to 7 psi. It
may release excessive pressure but should hold
between 1.5 and 5.5 psi pressure. Next, set the
tester to vacuum. If the one-way check valve
in the tank vent is working correctly, the vent
should not hold a vacuum.
“Was the fuel filter delivering fuel?” Use a ”
hook to remove the fuel filter from the tank.
See Figure 9-33. Remove the filter from the
line. Attach a vacuum tester to the fuel filter.
See Figure 9-34. If the filter does not hold a
vacuum, it was delivering fuel. If the filter holds
a vacuum, the filter is restricted and was not
delivering fuel.
Sheared integrated key
Goodheart-Willcox Publisher
Figure 9-30. A sheared key means that the
crankshaft quit rotating but the flywheel continued.
When the flywheel changes its position on the
crankshaft, the timing of the spark is incorrect.
The engine still has a spark but at the wrong time.
Tank should
hold pressure
Clamp on
return line Vent line
Fuel line
Goodheart-Willcox Publisher
Figure 9-31. With the cap in place and the
return fuel line clamped off, the tank should hold
pressure.