Call Champion at their 1800 number. I bought a Champion 3500w at tractor supply for $75 as is, because it wasn't making power. I called Champions 1800 number, they emailed me the troubleshooting and testing procedures. Found the problem to be a faulty auto voltage regulator, which Champion sold me for about $30 shipped to my door.
Not saying that is your issue, just making the point that Champion has pretty wonderful customer service , and they are very helpful when trying to diagnose things like this.
I have even had them send me parts for free before!