Yeah... it sounds like something mechanical or plumbing. That engine should run like a swiss-watch... even inverted. Check for kinks, or bends in the fuel line, and as someone mentioned the dreaded air-leak... Make sure you're carb is seated on the engine snuggly and that O-ring is perfect... Make sure there's no obstruction in the line or inlet to the engine.. I do believe that to be a "remote needle" set up.. I've been suspicious of the fittings and knuckle set up those use from day one... they seem like just one more turn/curve the fuel has to take before it hits the engine.... its usually the first place I'd look.. So...

Tuning an inverted engine can be somewhat sensitive.. but what you're describing sounds way out of any normal tuning.. there's a problem. Haha.. you knew that though.. I hope you get it worked out soon. Good luck.