Everything I run is inverted. They're a little more touchy, but they will run fine. I have a couple of thoughts...

Fuel spray will work its way to the glow plug more quickly in an inverted engine and snuff out the fire if the mixture is too rich, or the plug is old or the wrong one. If you aren't running a new OS #8 in that engine, try it. It's an expensive plug, but may be worth it. This may help.

Also, if you can't adjust it out, and you can't move the tank for some reason, there is always a Cline regulator which will most likely fix it. At $50, it's a little expensive for a small plane, but I can tell you that once set up the engine will run awesome. I have never had a single deadstick with the Cline.

Good luck, Chuck