I made one gasalternator for a few years ago, it was made with a B&S 3,5 hp gasoline engine and a bosch 35A alternator. the engine and the alternator was bolted in to a steel frame, at once i engaded (power to fieldcoils) the alternator the engine died, it was not lack of power, it was someting strange with the ignition system on the B&S that did not could stand the alternator when it was charging, the solution was to insolate the engine frome the steelframe, dont ask why, it was fine for me when it was charging :-)