engine won't stay running?

I have a 6.25 hp briggs and stratton engine on my neighbor's mower I'm trying to fix for him. I can prime it fine and it will start, only to run 3 seconds then die. I can push the prime bulb 3 times and it will fire again, only to die after 3 seconds. I cleaned the carburetor out with carb cleaner and it looks clean, air filter is ok, fuel line is ok, I'm assuming the spark plug is ok because it will fire initially.
thanks for any advice