If you have not done so I would keep water away from any internal parts as I think it will compound the problem, I can see where the writer is coming from though.
I have a similar problem on my EF1000is, exactly the same symptoms. I was really lost until the specialist generating company I bought it from told what to do. Which was to unplug the connectors from the stator, if it pulls over freely the stator has failed. They don’t know why this happens. I simply turned mine off after working fine, on restarting it struggled to run and then stopped, he said this is not uncommon on this machine. As you pull it over the coils produce a magnetic field opposing the direction you are pulling, hence the harder you pull the worse it is, by disconnecting the wiring this is stopped. He says that this problem doesn’t cause any other damage, so replacing ithe stator should do the trick.
I have yet to try, read on.
If it is the Stator Ass, I believe the USA part number is 7VV 871501000 or 7VV 87-150-10-00 , but check, and I have found them on the internet in the states new for as little as $165. by googling the part number
This doesn’t help me as I am in the UK and Yamaha won’t tell me whether this is compatible with the UK model as the output voltage is 240 volts. But I think it is the inverter that determines the output not the stator, either way they tell me. So I am looking at $307 here! I am going to see if I can get it rewound before paying that.
I hope this has been of use to you, but remember I have not yet put it to the test. Let me know how you get on, I’ll do the same.