Onan Gen Set??
 

I Have A Gen. That Will Only Stay Running When Holding The Start Button In.as Soon As I Let It Go It Stops. Any Ideas?? Anybody!!!

Low oil pressure over-ride maybe? Some generators have a built in relay that overrides sensors for start up. Can you get to the back of the panel?

What model number?

If it is not behaving like normal, suggest you get a cheap oil pressure gage from Advanced Auto (pepboys, etc) and see if you are getting normal pressure upon startup. Don't keep holding that switch down or you will sustain major bottom end damage if something is screwed up with your oil system. Check to see that you are getting adequate oil pressure and if you are, then your sender could be faulty or an overtemp thermal switch could be faulty...or the wiring to any of these safety switches has a bad connection...these safety switches are usually in series so if anything fails, it will shutdown.

Some of the onan gen sets have fuel pump issues and behaved just like you discribed. Check your fuel flow and if possible the pressure. Is it a 6.5 in an enclosure?