I'm trying to help the no load hz looks good rpm good. Is there a way to to check fuel delivery when you load it . I mess with a few generators and sometimes under a full load I have to open the main jet a little bit even though its fine with no load. But I don't know diesels to good but I guess it could be fuel delivery. Is there a good way to clean the injectors or any kind of setting on the pump that you know how to do without causing problems. Maybe some linkage hanging when it tries to load and ask for fuel. I'm just trying to think of the simple stuff . When you load it try to watch your linkage or move it by hand see how it sounds and feels.