The manual crashed my Adobe plugin reader. No loss, I doubt that I need to read the service manual that likely says that a Walbro carb is an all position design as I'll stipulate that.

If you read my full post, it noted that I've many gas airplanes and add that most of them run Walbro or Walbro knockoff carbs and I've only one of the many that richens when inverted. What I know for a fact... is that it does that. I also know that there must be a cause.

One more time... until someone provides me with a better theory, my belief is that gravity plays an effect with the metering circuit on that carb. The atmospheric diaphragm has the task of providing a constant to wit the needles can be set. If the needles do not hold a mixture setting, then it likely is that the metering circuit is varying. As Av8tor outlines, varying ram pressures in the cowl can do that but simply inverting the aircraft doesn't change ram pressures. What does change is the gravity effect upon the components of the metering circuit and albeit they are designed not to be influenced by gravity and evidently most often are not as evidenced by my many planes that will invert with no issues, I've one plane that does have an inverted issue.

What would you have me to do... tell myself that the plane is not richening when inverted? How do I go about persuading other observers that they are also mistaken?