Check any restriction in the vent line that may create pressure inside the tank while fueling.
Try keeping the throttle trim all the way down (minimum) after fueling and before starting, so the low neddle helps blocking the fuel.

Copied from http://saito-engines.info/faq.html:

Q. I want to install my engine inverted. The guys in my club say you need to install an on-board glow driver for inverted 4-stroke engines. Is this correct?

A. There is no need to use an on-board glow driver if the idle mixture is set correctly, you are using an appropriate brand glow-plug, and proper fuel. The idle speed will be reliable and just as low as for an upright engine.
Make sure the tank centerline is about level with the carb or a little lower; inverted engine problems are often related to the tank position being too high.