The charging system produces unregulated voltage and the engine only produces it's 5 amp maximum output at 5000 - 6000 RPM. The problem is an application error from using too high of a pitch propeller for the load. To achieve the best possible performance from that engine you will need to drop down in pitch. There are two different 6 pitch props available for that model engine. I would suggest going to the following one as it sounds like you are far below the max RPM.