Running 100% gas (not that ethanol mess) in it the last few minutes will do wonders as the gas will not gum up as bad as the ethanol mix.
If you pull off the vehicle gas tank then this requires installing a valve so you can switch to an external tank (or a jar with a little gas in it).
The gas additive Stabil will keep gas from going bad as fast.
They say the generator head needs to be exercised monthly. This is good advice. However, I just revived a 4000 onan that sat for over 10 years. All I did was change the oil the fuel filter and replace the carb with a $30 ebay unit and it ran. It smoked a little at first, then surged a little, but ran good after a few mins.
Yes, you can still get carbs, and they are not $300, they are $30.
I could have cleaned out the original carb as it would run but I think the float in the carb was not acting right and would flood. That was just a matter of cleaning the carb out better... Still, easier for me to just get a new one. I kept the old carb as the new one is probably china garbage.