Hello how are you
Your gen set is exibiting classic signs of a dirty carb.
At this point you will need to remove the carb, completley disassemble it, and soak the components in a quality carb cleaning solution overnight, then blow everything out with compressed air. That should get you back up and running properly again
If you have any further questions please post them here and i will help you any way i can.
Please do not base feedback left for me on the system error we experienced.
The voltage output of that set is based on engine RPM. 3600 is the proper run speed. You will need to get the rpms down to properly diag the generator. As far as should the engine return to idle with no load, i will need to look through my older manuals and get back to you. This is a 20 year old unit. Did not what to keep you waiting longer. I will post my findings shortly.
It appears that model runs at a constant 3600 rpm. Clean out the carb. Be sure to poke though the jets with a piece of soft metal, use a metal bread tie, this insures they are completley clean. I will also post you a link to the manual if you do not have one already.