High voltage from DeVilbiss GBV7000

I have an old generator with a Briggs & Stratton 294447-1042-A1 engine. It's putting out 150/300v instead of 120/240. I can't find much documentation online for it (service manuals, etc), but from reading around the Internet it sounds like the engine speed controls that. Does anyone know how to verify and/or adjust that for this type of generator?