My 1950 3600 would do the same thing. I fixed it by putting a fuel filter upstream from the pump. It seems crud from the tank was getting into the pump allowing it to pump fuel in both directions. The amount going to the engine was not enough to keep it running longer than 25 minutes or so. I betcha I had the carb off and torn apart at least a half a dozen times trying to fix the problem with no luck. I do have a glass bowl filter at the carb and noticed that when the engine would quit, there was very little gas visible in the bowl. When everything was working, the bowl would be about 3/4 full.
