On Friday, November 10, 2017 at 9:44:07 AM UTC-5, Roberto wrote:
Storaged for 3 years. Start and run perfect but for few seconds only. Oil
level is ok so the spark plugs.

--
for full context, visit https://www.homeownershub.com/mainte...g-1150714-.htm

Can't help with the manual, but a good place to start with diagnosing is
to put a voltmeter on it and see if it's putting out voltage during the
brief period it runs. If it's not, the controller will shut it down.
No voltage, you know it's a generator section problem, not the engine.
If you see normal voltage, then it's most likely an engine problem. I
had one with a shot rotor that did what you describe. From what I've
seen, these Generac's are a disaster. The reviews on Amazon were terrible
few years ago when I last looked.

Did you try googling for the service manual for that model?