How can I fixed this voltage output?

Get in touch with these guys: http://www.generatorguru.com/
They will guide you through it. FREE!

Sanders
November 2015

Clearly there's a problem - if you have warranty then invoke it with the vendor.

If you have no warranty because you bought it second user perhaps speak to the person who sold it to you.

if you're forced to make the repair yourself you will need test equipment - a small multimeter as a minimum.

Might it be something as simple as a wire fallen off - or a defective breaker? Might the generating engine be defective?

The answer is down to you.

Good luck...
April 2015