If your Generator has surging or hunting problems that can be traced to the fuel system, the root cause is probably not a warrantable failure. Component failures due to Fuel Varnishing are not covered under warranty.
If the Generator is not operated often enough to run a number of gallons of gas through it every month, it will start to develop Fuel Varnish. Emptying the tank will not prevent varnishing, because a small amount of fuel always remains in the gen-set fuel system, ie the carburetor. A slight decrease in efficiency can have a huge impact on performance. The generator may start but sometimes will surge or hunt at various loads. It will surge or oscillate between acceleration or deceleration of the engine speed, and never settle on a certain operating speed. Speed affects the output voltage so any surging greatly hampers the gen-set from providing stable electricity.
See your operator's Manual under the section "Generator Set Exercise", it states the set should be run with at least a 50% load (AC ON) for two hours every four weeks.
Onan also recommends the use of a Fuel Stabilizer (see Fuel Stabilizer)