Based on your original post, you could reach 2330W before the engine would start to stall. Considering the engine should maintain 3500W, there is at least 1200W being lost somewhere.
Gdetrailer provided a sound explanation of how a shorted generator winding would perform and your tests support this. There is at least a 5V delta between windings and providing a secondary fuel source did not improve performance.
So, is the lost 1200W being dissipated in the generator head, as Gdetrailer referred to as preload? If so, I would suspect the generator would become hot. Maybe your next test would be to use a single 1500W heater as a load and see how long it takes for the generator head to become hot to the touch.
