It sounds as if you are over-compounded. Have you looked at the series field shunts? On an Otis controller, the series field is shunted with that coil spring-looking thing in the back of the controller. In your situation, you probably are using some german silver shunts in the series field. Also with the Gennie at idle with no demand, you should not have much more than 5 to 7vdc, measured at the brushes. If it is much higher than that you are prob out of neutral.