Equalizing your Walbro Carb

On rare occasion  you will find that your Walbro carb changes tune when you install your cowling or when you fly your airplane. This is a common problem on many airplanes. What happens is, the air flow going into the cowling pressurizes the cowling. This air pressure pushes on the float diaphragm which in turn lets more fuel into the carb. This phenomia causes the engine to run rich. There is a vent located on the diaphragm cover which  is intended to equalize the float diaphragm at atmospheric pressure. Anytime this vent sees air pressure other than atmospheric pressure, the carb WILL run rich.

There are a few things you can try to correct the problem. First, I would rotate the vent on the carb. Sometimes this will cure the problem. If this doesn't work, you will need to connect a line to the vent and route the line to a more stable environment. The "ideal" environment is inside your fuselage, so thats where you want  the new line to go.     

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Remove the cover and solder a brass tube on the existing vent. Be sure the tube is OPEN to the inside of the cover! Attach a line to the new tube and route the line to go inside the fuselage. This will COMPLETELY resolve pressure issues on the diaphragm. That's not to say you don't have other problems, but the atmospheric problem will be gone.