A little help for my son's generator

[jsb5717]

Just checking the electrical expertise here.

My son has a 30 amp TT. He has a generator that should be adequate but it keeps popping the breaker when the AC is turned on. There are 2 settings for the 30 amp plug...either 120v or 240v. He's using the 120v but it won't work.

This is his generator. Any thoughts?

http://www.ngcindustries.com/11-8120.html

Thanks for the help