I have an Onan 2.8 KW generator in a 1994 RoadTrek that hasn't been run much.
With a 1/4th tank, I added "Seafoam" added 5 gallons then ran it for about an hour.
It runs with this rhythmic surging... reduces a bit with a load (electric box heater 1.5KW)
To clearly show this, I made this short 37second Youtube video to save on "a thousand words" to show what is happening.
Notice when I push on the choke lever part way that the engine smooths out. Push too far and it floods the engine causing dark exhaust smoke, back off and it runs smooth. Let go and its back to rhythmic surging. BTW I am 200 feet above sea level and the temperature now is 38F.
How can I fix this?