I tend to agree on the carb. If any type of gasket needs to be replaced, make sure it gets installed the correct way. Sometimes is hard to tell, I learned the expensive way to the tune of $450 on a boat generator having the same symptoms. Had to call a factory tech who knew exactly what it was. Reversed the gasket.
It's funny, when I had genny problems while on the road, a repair shop offered me help as long as it wasn't an Onan. He said he refuses to work on them any more. He gladly worked on my Honda which I always carry as my backup genny and has become my primary unit to take care of my boondocking charging needs because it's more quiet. $35 and 45 minutes for the repair.
I do alternate running both genny's just to keep the fuel in circulation.