Sounds to me like you've taken care of the tank height issue, which should be centered at carb inlet level. With that out of the way, here's my take on the situation.

With engine inverted, fuel is pooling or building up in your glow plug; not with engine upright. Fuel presence at the glow plug cools it. To confirm, try idling with glow driver on, should improve.

If so, reduce the fuel flow into the engine at idle with the low-speed metering adjustment. On an FX, that's in the carburetor shaft, and on an air-bleed carburetor it's on the carb body next to the throat. With a leaner mixture the plug should stay hot & ready to go at idle.

I have experienced 3 engines which needed a larger air bleed hole to avoid the problem I described. Two were Tower .40's, one was an OS .25 FP. One numbered drill size larger than the existing air bleed hole was what they required. I mention this just in case you have an air bleed carburetor.

Good luck,
Dave Olson