How proficient are you about taking the carburetor off and taking the main, pilot and idle jets out for an examination and cleaning? I went through this exercise with my brothers Champion 3100 which was surging at idle in a fairly pronounced fashion. I chatted with customer service three times and they were very decent to deal with. The ended up sending us a brand new carb which i swapped out for the old one. The unit ran spot on after changing out the carb. I would call Champion and see what they want to do. The 3100 is a fantastic generator.
Cheers!
Mike