Problem with Generac Generator

I just bought a used Lance 1130. P.O said the generator ran a little rough. I got it home and it starts right up and I turned on the air conditioner to test it and it worked fine. No problems until I turned the AC off. Then the generator accelerated wide open for a few seconds and shut off. I tried this three times with the same result. Any ideas. Thanks in advance for all help. Carey
BTW: This is a Generac NP34 LP. Model 803-4