Lawn mower won't stay running?

I have a Briggs and Stratten 6.75 Quantum Lawn Mower that is 4 years old. I put a new spark plug, Air Filter and changed the oil. At first it wouldn't start at all then it ran for 1 minute and died, now after 25 Pull start tries it will run for 1-2 seconds and dies. I am assuming it is from the gas form the 4 pushed of the primer ball. What could be causing it to not stay running?