I have my DLE 55 in a 30% EG Raven. I'm running a 24 oz tank that is right on carb centerline. The problem is it runs perfect right side up but when I go inverted it immediately goes very rich in the midrange. Flip it back upright and it leans back out again.

With the pump carb I can't understand why this is happening. Has anybody experienced this?

Pretty plane! I had a Global kit years ago...

As you say, there shouldn't be any difference and the gas tank doesn't need to be on centerline of the carb. I wonder if your carb diaphram is bad.

Might be a low pop off pressure problem?

It could be a pressure change within the cowl when inverted that is affecting the diaghram vent on the carb .
I would remove the carb diaghram cover and solder a tube to it and run a line from the vent cover to inside the fuselage (away from large sudden pressure changes).
There are several threads on installing the vent tube and line here on RCU.

I'm thinking the pressure doesn't change much while inverted - the plane tracks like an arrow with almost zero pitch attitude change.

I'm inclined to agree there's something wrong with my carb, even though the engine is VERY low time. Where can I get a rebuild kit?

Your local lawn mower shop