by Mike Forwood - Australia
Most of the pilots who use a membrane carburetor noticed how hard it is to prime it without pushing the membrane. Some of them use a cable tie fastened close to the carburetor.
Others, drill a bigger hole in the membrane plate so they can prime using a finger. Enlarging the hole may allow dust or sand particles to get inside and cause a premature membrane failure.
The need to push the membrane is caused by a high pop-off pressure, sometimes too high, especially on the Walbro 32 carbs.
I agree with Alex Varv who told me that if the pop-off pressure is lower, squeezing the primer bulb hard enough will allow filling the carb with gas without pushing the membrane.
However, I know that not many pilots really check their pop-off pressure and this is why I built a very easy and convenient priming device. The picture below, illustrates all the parts needed:
You simply take off the metal cover on the metering diaphragm side, get a long shank pop rivet 2.6 mm diameter . The shank needs to be 20 mm long .
Insert the rivet from the diaphragm side ,poking through .
Remove the spring from a ball pen and cut it to the pop rivet shank length.
Place this spring on the rivet that is poking through the plate , and put a washer on and holding the washer down a little, use a pair of pop rivet pliers to slightly pull the rivet up about 2-3 mm so there is pressure on the spring .
Now comes the most important part.
Place this whole assembly between a set of vise jaws with the rivet nail facing up and tap the nail out of the rivet ,being careful that you do not damage anything. Removing the nail gives a breather hole so the diaphragm can work correctly and the spring stops the rivet from rattling around and also, holds it off the diaphragm.
By installing this, it takes the pressure off the rubber diaphragm when priming the motor thus stopping it getting stretched and not letting enough fuel through at max throttle.
Regards Mike Forwood
Note: here is a picture we received fromMike Masterson (California) who also built a push primer fro his carburetor. This device is even simpler and works great.
The "primer consists of an ellastic arm that has a small pop rivet installed. When pushed, the rivet presses against the diaphragm thus priming the carburetor.