Can get generator to start but will not keep running more than a few seconds, any ideas on what is wrong??

I have a Coleman Vantage 8K watt generator w/18hp B&S electric start engine, that will start-up but won't keep running. I've determined that it's getting air and spark and getting fuel from the tank to past the inline filter. It won't start w/o choke and then will fire right away when choke is applied but dies within a second or two whether you turn the choke off or not. This generator was left behind from the previous owner's of the house and has sat for over 2 years w/o fuel in it or w/o being started. I have filled with fresh fuel, checked the oil level and condition, wired to use pick-up battery to start, and cleaned air filter. This generator is used for back-up power to my home and temporary power in my barn and is mounted in the bed of an old p/u that I use on the ranch. Any ideas or suggestions will be greatly appreciated......