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.
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.
Carburetor leak detector also for popoff pressure testing. It requires adding some way to pump up the air pressure like a squeeze bulb.
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.
POPG Pop-off pressure tester
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.