Onan Marquis Gold 5500 shuts down after 20 minutes

mustangglp wrote:
I would ck the vent for the gas tank? Take the gas cap off and see if it still does it.

that is a good start, another idea would be, let it run until it stalls and immediately try to restart it, if it won't start, use some starting fluid (after you remove the air filter), if it runs for a second or so, its a fuel problem. Go easy on the starting fluid, I overdid it one time and she backfired and burnt the hair off my arm.