I have a Briggs and Stratton 450 series engine on a push along mower. 9T502 0134 B1. It starts but refuses to run. It will start on the first or second pull, then runs for 2 or 3 seconds. It stops dead and refuses to restart at all. If I come back to it a few hours later it does exactly the same thing. I have fitted a new air filter, spark plug, carburettor gasket and diaphragm and the coil pack. The plug gap and coil air gap are correct. Help please.

Garden - Briggs