If the fuel filtration has been cleaned and the spark plug has been ruled out, it's time to get into the carburetor.
It sounds like it runs fine at wide open throttle (eco off) but stalls at part throttle. This sounds like a clogged transfer passage inside the carb body.
I'd drain the float bowl and remove it w/ the carb in place. There's enough room inside the EU3000is for this, and it will let you examine the bowl for signs of corrosion and/or contamination, sludge, anything else that doesn't belong there. If you find stuff, clean it out. Spray out the main venturi tube with carb cleaner, shove the can straw up the venturi and blow it out. Remove the float hinge pin, bring out the float and valve for inspection. Spray out the valve base with carb cleaner. Reassemble the float, the bowl (careful to align the rubber o-ring that seals it to the carb body), and re-test. It may fix it completely, may alter the symptoms, or may have no effect at all.
If this doesn't reveal and fix the problem, it's rebuild or replace the carb. You'll need an air compressor and rubber-tip nozzle to do a decent cleaning, and a solvent tank to do a really thorough cleaning.