Toro timecutter z420

I bought a used toro timecutter z420 it had been running great I was mowing when all of a sudden the motor ( B&S intec ) stalled I could not restart
The first thing I did was change the spark plug it started fine it ran for a time then it did the same thing it sputtered an died I checked the plug and it was fouled I took the carb off and checked it out it looked ok and I put it back tried to start the motor and it started up
I mowed for about 15 min and it did the same thing sputtered an died
Thank You
John L.

Next time it starts to die out, quickly open the gas cap and see if it comes back to life. If it does, time to replace the gas cap, as the vent is plugged. If there is no change, the next thing I would check is the fuel pump. When you report back, please include all engine numbers.