Hi,
Keep running new gas through that generator. Eventually you will get out all of the varnish, and it should start to run better. You can take off the air filter, and try spraying in some carb cleaner while it is running and under load.
You can also try increasing the load on the generator by running one A/C (1400 watts) and a electric heater. You can start it on low - 800 watts, then change to high - 1500 watts. If it will run both, try shutting off the heater at the same instant as the second A/C starts, even though it will add a surge start load (about 2,500 watts at start up) it will be shedding a 1500 watt heater load so the throttle should be nearly full open when the A/C starts.
Good luck!
Fred.