Why does my lawnmower start and die even with a new carburetor?

We have a Yard Machine push lawnmower with a 09p702-0004-F1 Briggs and Straton engine on it. We got the lawn mowed once this spring with it, but since then it has refused to run. It turns over, starts up and then dies.

After researching on the internet, we cleaned the carb with some carb cleaner. If you spray carb cleaner in it started and ran as you kept on spraying carb cleaner in. We took apart, then reassembled the carb. That didn’t help. We checked the fuel line - fuel flows freely out of it. Drained all of the fuel out, put in new fuel. Nope. Tried it with the gas tank cap off. Nada. Constantly pumping the primer once it starts doesn’t seem to help at all. Cleaned the spark plug.

We then ordered a replacement carburetor. Installed that today - still no go. It continues to turn over, start and then die immediate.

Where to go from here? The lawn grows longer by the day - we resorted to using the string trimmer on it last week . . .