My 4000 with a 172 gasser will run for about 5 minutes at a time and then sputters and dies. If I mess with the throttle, I can keep it running but it runs rough and won't come back to high idle before it dies. I replaced the carb, and rerouted the fuel line. When I bought the tractor it had the fuel line running between the head and exhaust manifold. I thought it might be vapor locking so i rerouted it. I hooked up a timing light to all the plug wires, it's not losing spark. About 30 seconds after it dies, you can hear gurgling through the intake. If I remove the fuel cap, air seems to be purging back into the tank. I'm out of ideas. Any one have some?