ORIGINAL: Nosedragger
You can't set needles with an expansion chamber on the ground and expect it to work in the air.When the prop unloads and rpm rises the mixture will be wrong.
I'll add that among my several gas planes, I've one that goes richer inverted. I suspect the cause is gravity influence on the atmospheric diaphragm. It is a rear carb 35cc and the diaphragm is on the bottom of the carb when upright. I suspect that the little spring is slightly weak and that when inverted, the weight of the diaphragm opens the float needle slightly more. If/when it gets rebuilt, I'll substitute a slightly stronger spring and see what happens.
That is dead wrong. The Walbro is an ALL ANGLE carburetor.Get Walbro document C-1022.
Could this be a problem with the membrane of the carb being old and not flexible enought which is allowing to much gas to the carb when inverted?
you probably answered your own question, replace the metering diaphragm and the pump diaphragm.
I'm all ears if you have a more likely explanation... otherwise, that is my theory until I hear a better one.