Re: Onan OBFA-IR/16004C RV 4.0 Generator Manual
Kpack just posted links where you can download most of the manuals for your generator. The Operator's manual covers operation and general maintenance, the Service manual covers in-depth repairs, and the Parts manual gives you the part numbers you'll need to find replacement parts. Just grab them all. The only manual Kpack didn't include was the 965-0612 Installation manual but it isn't too likely to be useful unless you just want to complete the set. But neither of us have that one.
We don't recommend trying to tune the carb by ear. The generator wants to run at 1800 RPM all the time and tuning the carb is a matter of juggling carb adjustments and governor adjustments according to information in the manuals. Before you get out the magic screwdriver, you might be further ahead to try running it on an alternate gas can fuel source and fill it with non-oxygenated (no alcohol) gas and a heavy dose of Seafoam. Run through a gallon or two of that and see if it runs better.
If you do still end up rebuilding the carb and need to tune it, then you might want to look for a "Kill-A-Watt" meter in the electrical section at your local hardware or department store. They usually sell in the $20-$30 range, closer to $20 with coupons from Harbor Freight or by purchasing online from the Home Depot web page. It is even cheaper at Fry's Electronics if you have one in your area.
We recommend the Kill-A-Watt because it plugs into an outlet and gives an easy to read digital display of voltage and frequency, the two items you need to monitor while trying to adjust your generator. If you really want to know RPM, just remember than 60Hz times 30 equals 1800 RPM, so 62.0Hz equals 1860 RPM. Good luck!