If you're looking for a manual for an Onan 4.0kw gas genset generator, you've a few options as to where you might find one.
One place you could ask is the manufacturer: Onan products are now made by a company called Cummins Power Systems, who have 10 branches in 6 states in the north-eastern US. You call call one of these branches for help, or send an email via the company's website.
There are also sites devoted to old engines and generators, such as smokstak.com, who might be able to offer advice.
Lastly, there are of course the usual auction sites, such as eBay, where old manual often come up for sale. You may find someone selling Onan products who could help with manuals or provide advice.
YouTube is another place where you can pick up tips on troublesome generators. Try this video and see if it helps:
