1974 Volkswagen Beetle car starts but suddenly turns off.

The problem im having with my bug is that it starts and it suddenly shuts off. I thought it was bad wiring but I checked it all and its ok. The previous owner installed a push start button that is why I thought it was the wiring. Spark plugs, wires, rotor and points all are new. I was told it could be the fuel pump so I changed the fuel pump and filter but it still has the problem. Sometimes the car runs for a couple seconds and turns off sometimes it runs longer, but it always turns off by itself. Please help me I cant figure out what the problem is. Thank you very much in advance for your help and time.