A gallon through the DLE 20 is probably at least 2 hours...maybe more....I would be looking at how long the engine has been sitting without running. Fuel can gum in the carb pretty easily, and if not very well-filtered/clean fuel, the previous suggestion of the carb screen would be my first stop also. Because it starts and runs, pretty unlikely that ignition is an issue. Pull the spark plug, make sure it's not fouled, but probably is not. Highly likely you are dealing with carb issue, because the 2 or 3 seconds is probably the prime run burning off, then the carb isn't pumping to provide more.