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CHAPTER 9 Two-Stroke Engine Troubleshooting and Evaluation
Copyright Goodheart-Willcox Co., Inc.
the purge prime system, including the purge
prime check valves, is working. If the vacuum
gauge does not show a vacuum or if the gauge
drops slowly, there is an internal leak inside the
carburetor, past the accelerator O-ring or the
metering chamber check valves.
If the pressure drops when the fuel line, car-
buretor fuel pump, and needle are tested, remove
the fuel line at the carburetor. Attach the pres-
sure tester to the inlet fuel fitting of the carbu-
retor and pressurize the pump to 7 to 10 psi. If
the pressure holds, the fuel line was leaking.
Continue by performing the purge prime sys-
tem test, releasing the bulb several times, and
observing if the needle drops in increments.
If the pressure drops when testing with the
pressure/vacuum tester connected directly to
the carburetor’s inlet fuel fitting, the carburetor
will need to be examined for leaks when it is
removed. The possible sources of leaks are the
purge prime check valves, carburetor fuel pump
system, or the carburetor’s internal fuel delivery
check valves. Remove the carburetor.
Find the carburetor leak by attaching the
pressure tester to the carburetor inlet fuel fit-
ting and adding pressure from the pressure tes-
ter. Lower the carburetor in a clear container
filled with water. See Figure 9-37. Identify on
the carburetor where the bubbles are escaping.
The carburetor in Figure 9-37 is leaking around
the fuel pump cover. The cover could be warped
or the gasket or diaphragm could be leaking.
A B
Depress the bulb Pressure holds Release the bulb Pressure drops
Goodheart-Willcox Publisher
Figure 9-36. Testing the purge prime system. A—With pressure on the gauge, press and release the
primer bulb several times. B—The pressure gauge needle should drop slightly each time the bulb is released.
Bubbles from
pump gasket
Goodheart-Willcox Publisher
Figure 9-37. Pressurize the carburetor at the
inlet fuel fitting. If pressure leaks down, lower the
carburetor into water. Bubbles indicate where
there is a leak in the carburetor.