Generator overloads with A/C

I know this topic has been beaten to death, but I am at a loss. My Champion 3100 immediately overloads when the a/c is turned on. The a/c unit is 13.5K BTU, so the gen should handle it. My older gen has a higher capacity. It would start up the compressor. After a minute or two, it would start to motorboat then trip the 30A breaker. I can run the a/c connected to the house on a 20A circuit with no problem. OK, to mitigate the normal questions:

Refer - off
Microwave - off
Water pump - off
Water heater (elec) - off
I even shut off the converter

What am I missing here?

- Jeff