Newbie, Please Help, does anyone have ideas ??

7K model 4389-2 running on natural gas. Generator has been running good for about 5 years with about 50 hours runtime. Last month, my area had severe T-storms which knock out power for 5 days. I notice the Generator was not running. I opened the top and could not see any problem, the switch was set to auto. I turn the switch to manual start and it will run very rough for about 10-15 seconds then shut down. I repeated the manual start one more time and got the same result.

Last full maintenance in Aug 2010, I replaced my battery last December.

Steps you have taken to resolve problem:
Changed spark plug, air filter, oil and filter.
Checked air intake and Bellows, both seem to be fine.

Still have same problem, Any ideas on what I can do?