Onan 4000 generator runs rough under a load

First, check the obvious like the fuel and air filters.

But the most likely culprit is dirty jets in the carburetor, caused by not running the generator frequently enough. The solution is to replace or rebuild the carb.

You might find this thread interesting:

https://www.smokstak.com/forum/showthread.php?t=181300

That forum in particular is very good - it has links to the service manuals for your generator that includes detailed diagnostic and repair procedures, far beyond what is in the usual customer manuals.

Since your genny has probably been not getting enough exercise, you might also want to check the resistance between your two brushes. Should be less than 80 Ohms, ideally less than 30. High resistance means you need to clean your slip rings. Flight Systems makes a "Slip Stick" for carefully polishing the rings while the genny is running/cranking. But you can also make your own with a dowel rod and some brown 3M scrubby material glued to it. High slip ring resistance will damage your voltage regulator ($250+).

Seafoam and other "mechanic in a can" solutions probably are not going to help, but it's worth a shot.

Is there someplace you can take it in for service? Over the years I have had several people complain to me about hard to change fuel filters. They changed the original filter so no adaptation was required then mounted a giant filter inside a frame rail. Problem solved forever.

gbopp,

Thanks for the feedback. I will try the spray seafoam to see if that does the trick.

I have not replaced the fuel filter and hesitate to get into all the disassembly that it entails as I am not very mechanically talented. :(

When was the last time you changed the fuel filter?
My Onan surged at idle. I got a can of spray Seafoam and sprayed it into the carburetor intake while it was running. Then I shut it off and left it sit overnight.
The next day it idled properly, no more problems.