My buddy had the same problem with his Yamaha EF2000 and trying to start his PDI9260 charger. The camper next door came over and offered him his Honda eu2000i - which had no problem with the load.
When he got home, I brought my eu2000i over and we did a number of tests starting various high load items (AC, toaster oven) with both gens. Each and every time my Honda would start them without struggling, even on Eco, but the Yamaha wouldn't do it.
The Honda's are known for their extreme surge capacity. They've been measured putting out twice their rated current.
The Yamaha just can't do the same.
