Pretty sure that motor uses a rectifier only. So higher voltages are normal make sure you are using a good non sealed standard battery. That style rectifier uses the battery as the regulator so you need to be able to refill it.
You do realize how old that 4 cylinder design is don't you? And hard to believe it only used 3 gallons of gas in 4hrs of hard running. If your top Rpm at full throttle is only 4700-4800 rpm you might need a different prop.
