Your generator should run at a level and constant RPM. If it revs up for a moment and then ruffly idles down and then revs up again and so on, it's surging or what we might call hunting. If it's surging, the electricity that it produces will not be constant which can effect the appliances in the living quarters. A good indication is to listen to the Roof Air or the furnace fan while they're running. They will surge with the generator.
If the Generator is surging to much, - when you turn the AC on the generator dies, as mine did. If yours dies, check the fault code. Mine was blinking a 14 which means, take it to an Onan Service Center.
More often than not the cause is a varnished up carburetor. Which means the fuel has set in the carburetor too long due to not running it often enough. Onan recommends running it 2 to 3 hours a month, or using a fuel stabilizer in the fuel to keep the fuel fresh.
The normal fix is replacing the carburetor.
But, you might try this first. It doesn't work that often but it might save you a trip to the service center and a couple of hundred dollars.
I can't say for sure which part of this fixed it, I just know this is what I did.
At the Parts store, get 1 Pint of "Sea Foam Motor Treatment for Gas and Diesel Engines". One Pint treats 8 to 25 gallons. I put in 8 oz (half the can) per 8 gallons.
At the Parts store, get one aerosol can of "Sea Foam Deep Creep" - and if the Air Cleaner hasn't been changed in a while, have one of those ready. (see Onan Part Numbers) in this section.
1st - Check the Oil to make sure it's ok, and then Remove the Air Cleaner. ( see photo A & B ) Take a rag and wipe out the air cleaner housing so no dirt gets sucked into the carburetor.
2nd - Start the Generator. With the Deep Creep, spray through the air cleaner housing into the carburetor for about 4 to 5 seconds. You can look in the housing to see if your spray is going into the throat of the carburetor. The generator should rev up and stabilize for a minute or so before it ruffly idles back down. Each time it idles back down, spray again for 4 to 5 seconds. Do that about 8 to 12 times.
3rd - Next, locate the Altitude Adjustment Knob on the carburetor. With the unit still running, use your index finger on both hands (see photo C & D) and move (rotate) the Altitude Adj Knob all the way in each direction, back n forth, back n forth - about 3 dozen times and stop with the knob indicator close to whatever your altitude is, 2,500 ft is about half way between 0 and 5,000. Doesn't have to be exact.
Hopefully at this point you'll notice your generator starting to run longer at the higher RPM without ruffly idling back down. If you do not notice a change or maybe you noticed a slight change but it's still not right, repeat everything again starting with spraying into the throat of the carburetor.
If you think it helped, PUT THE COVER DOOR BACK ON THE GENERATOR AND let the Generator run for an hour. After an hour, put the Air Cleaner back on. After another 30 min turn the AC on and leave everything on for about 2 hours.
If this made no difference at all, more than likely it's going to need a new carb, but a second opinion would be good. There is no cleaning the carburetor and they don't make kits for 'em.