200
Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
Test the purge prime system inlet and out-
let check valves by attaching the pressure
gauge to the return fitting on the purge block.
See Figure 9-38. Depress the purge bulb sev-
eral times to pressurize the gauge to 7 psi.
If the gauge can be pressurized, the two one-
way check valves in the purge prime system are
working correctly.
Evaluate the carburetor internally. Inspect
the fuel pump components. If the fuel pump
diaphragm is plastic, it should be pliable and
not cracked. If the diaphragm is rubber, the rub-
ber should not be stiff. The two one-way check
flaps should sit flat and not be curled. Inspect
the pump gasket for tears before removing it.
There should be no debris or varnish in the fuel
pump reservoir. Look for white corrosion on the
carburetor body. This would indicate aluminum
corrosion.
The fuel screen should be clean. Look for
debris that could be obstructing fuel flow.
Debris on the fuel screen can restrict fuel flow
and could cause the engine to run lean.
Inspect the metering system components.
The metering diaphragm is made of rubber and
should be very flexible and pliable. The ethanol
in fuel tends to cause the diaphragm to become
stiff. The lack of pliability causes the engine to
run lean. Inspect the aluminum surface of the
carburetor block on the metering side for signs
of corrosion, varnish, or debris.
Test the main nozzle check valve. Fuel should
flow out the main nozzle through the valve and
the valve should seat when the engine is operat-
ing at idle. Perform a pressure test on the orifice
where fuel enters the main fuel circuit from the
metering chamber. Hold the rubber tube from
the pressure tester firmly against the main fuel
delivery port, Figure 9-39. Pump the pressure
tester. The check valve should let pressure pass.
Put a vacuum tester on the orifice and draw a
vacuum. The valve should seat and hold a vac-
uum. If the valve does not hold a vacuum, the
carburetor will need to be replaced.
Identify where fuel enters the idle fuel cir-
cuit from the metering chamber. If the carbu-
retor has a purge priming system, there will be
an idle fuel check valve. Test the idle fuel check
valve using the pressure test and vacuum test,
the same tests used for the main nozzle check
valve. Some carburetors with purge prime sys-
tems may have a part load fuel system with a
part load check valve. Pressure test and vacuum
test the check valve. These valves are non-ser-
viceable. If they fail the pressure and vacuum
tests, the carburetor will need to be replaced.
Measure the metering lever height with the
appropriate tool. See Figure 9-40. If the meter-
ing lever height is low, it would cause the engine
Return
fitting
Depress
the bulb
Purge block
output builds
Purge
block
Goodheart-Willcox Publisher
Figure 9-38. Test the purge prime check valves
by attaching a pressure gauge to the return fit-
ting from the purge system. Depressing the bulb
should pressurize the gauge. If the gauge pres-
sure increases, both purge prime check valves
are working correctly.
Main fuel
passage
Goodheart-Willcox Publisher
Figure 9-39. Press the tube from the pressure
tester firmly against the fuel passage to the main
nozzle check valve. The valve should allow pres-
sure through. Use a vacuum tester to see if the
check valve will hold a vacuum.