The eu2000i's idle speed is 3,000 rpm with eco on and 4,300 rpm with eco off. Maximum engine speed is 5,000 rpm. On mine, with eco on, it hits 4,300 rpm at about 1,600 VA load. From fuzzy memory (i.e., don't count on it!), I think it begins lifting off of the eco on idle at about 400 VA. The rpm is controlled by a microprocessor, based I believe, on the DC input voltage to the inverter. If the alternator output were weak, that could explain the engine revving too high. On the other hand, if the output supports 1,600 VA for more than a few minutes, I'd conclude that the alternator is working just fine.