New Onan 4000 wont start

I assume it has spark because it starts, I would still pull a plug and see if the spark will jump at least 1/4 in to ground and is nice and bright to rule out ignition problems. I would then disconnect fuel line to carb remove air cleaner and spray gas or carb cleaner into carb and see if it stays running if so I would suspect a fuel delivery problem. I have had two intermittent problems that drove me crazy like that. One was just a bad 12v connection to the fuel pump, the other was the pick up tube going to the fuel tank. I am sure yours in not cracked or rotted but it can be loose do to someone not tightening the hose clamp. Also you can remove the jets and spray the holes. You manual will tell you how many turns to open each one as an initial setting. Being this new I would think it is something simple. Also try disconnecting the sensors to see if that helps. If you try these things and report back you will get better suggestions.