Another vote for using starting fluid or carburetor cleaner. Take the air cleaner off, open the throttle valve, and douse it with carb cleaner. I use Seafoam, Amsoil Powerfoam or Berryman's B12 personally. Douse it really well and then try to start it. Once you get it running, let it run for a while (putting a load on the generator would be a good idea), like at least 15 minutes or so, turn the fuel shutoff valve OFF while running (assuming that the shutoff valve is working) and let the engine quit by itself after burning the fuel in the carb. Then I would strongly recommend removing the fuel from the tank by disconnecting the shutoff valve from the line to the carb and then opening it into a container. After that, of course reconnect the shutoff valve to the carb line.
Since this is a generator I'm assuming you use only in emergency situations when the power goes out, this should keep it honest enough to fire up when you need it (after putting fresh fuel into the tank and opening the shutoff valve). I have a Honda EU2000 generator and I never store it with fuel in it. I always drain the fuel out of the carb and then siphon out the fuel from the tank. It's always worked when I've needed it (so far).
Also, I've heard these Letayar fuel shutoff valves at the link below are good if the one that came with your unit gives up the ghost:
https://www.amazon.com/gp/product/B0B6C1714D/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&th=1