Without good electrical measurements or swapping components everything is opinion. Mine is that there's something wrong with your generator, air conditioner, the interconnections, or you have an additional AC load you aren't aware of. I base my opinion on (1) My eu2000i started a 57915 with 100% reliability on 90+ days, and (2) It will start my current 15,000 btuh unit at at 70+ degrees and 3,500 feet, (3) Compared to the eu2000i, the 2400 VA Yamaha is "over engined", so I'd expect it to do better than a straight 20% improvement over the eu2000i.
