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Membrane carbs are sensitive to the pressure required to push the needle
valve off its seat to allow fuel into the chamber: pop-off pressure.
This tool makes it easy easy to do. Alex Varv also has instructions on
building a pressure tester but it's nice to be able to order one already
built such as the one tested here.
There is a fairly wide pressure allowance and that the what you
measure with the motor stopped will not be the same as with it running.
Since it would be dangerous otherwise, the motor must not be running.
The tester and instructions used below comes from
Ron at
www.ParamotorAir.com. These
instructions were included with the device as of July 3, 2007.
Paramotor Air Testing Instructions:
We will use a Pop off pressure gauge for this (POPG). Remove the side
carb cover, the side that houses the spring & fulcrum lever. Slide the
membrane off the fulcrum. The membrane has a nub in the inside center
that engages into the fork of the fulcrum lever. It can be modified by
removing it. Snip the nub off and file it down flat it is not needed, if
you want.
Before testing we must make sure that the carburetor is "wet" inside.
Spray some WD40 into the output tube of the POPG. Connect POPG output
tube to the fuel inlet of the carburetor.
Close the valve on the rubber bulb by turning it all the way clockwise.
Squeeze the rubber bulb to increase pressure—an overhand grasp with the
palm is easier than just using the tips of your fingers. When the
pressure suddenly drops, that's your pop off pressure. It means
the needle valve, hooked to the forked end of the fulcrum, opened.
Different carbs require different pop off pressures. For the WB37 if
that pressure is over 12-13 it will be harder to start. I wouldn’t let
the pressure go below 11 PSI or it may run too rich. On my WB37 it was
originally at 19 PSI, which caused it to run lean and harder to start.
To lower the pressure remove the fulcrum lever screw and remove
the fulcrum and then the spring. Snip off one and a half rings of the
spring at a time and replace with the spring in place with the clipped
end down. Assemble the fulcrum again and test it for pop off again.
Check it several times. Continue snipping the spring little by little
until you get to the required pop off pressure. Tip: To release the
pressure in the carb, turn the valve on the bulb counter-clockwise.
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The Pop Off Pressure Guage makes the measurement
easy.
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