I have briggs and stratton 4.5 engine lawnmower. It has gas and oil. The mower starts for about 5 seconds and stops. I have lost the manual and I am out of money to fix it. Does anyone have a clue as to what might be wrong. Thank you.

Have you checked the fuel filter? How about the air filter? If the air filter is clogged, it will die because it is not getting enough air for combustion. If the fuel filter is clogged, when it starts to pick up fuel, it also picks up any debris in your tank and could become more clogged. If you smell fuel, it's not your filter, and probably the air filter. Try those first.:)

Could have moisture in the fuel.