Onan 5500 surging after warm

I think I would ask Cummins first also before I made any adjustments. I'm starting my new Onan once a month, as recommended by Cummins, and will be using Star Tron or some other similar fuel treatment for the ethanol in the fuel. I think most problems are from the lack of use.

Before doing anything else, I would recommend a healthy dose of carb cleaner in the gas. Berryman's B12 Chemtool is the best around, I think.
If you don't keep the carb clean (inside) you might end up needing a new carb every few years....to the tune of about $250 installed.

Also cleaning the linkage mentioned above.

Now might be a good time to get to know your local Onan dealer (Cummins).

Normally it would sound like a lean condition. It could also be a governor adjustment. I dont know that model but many small engines use very thin light springs that cover the throttle/governor links. These springs hook into the arms at each end of the link and keep the slop out of the linkage. If the springs are broke/disconnected it will usually"hunt" rather quickly but could also cause a rolling surging action.

It seems too new to have rotted fuel lines and sucking air. If adjusting the gov be very careful as some engines will expierence catastrophic damage if adjusted improperly. Not due if yours is mechanical linkage system or electronic. Time passes us by when our hands are no longer on the stuff.