Fire on the Prime and then nothing

I have a 20 year old B&S Toro lawnmower. Engine number is 12H807-0660. I'll use the prime bulb about 5 times, and then start the mower. It will fire on the prime and then die after about 3 seconds of running. I've gone through the carburetor and cleaned everything inside. All the jets appear to be clear. The float appears to be working correctly. Fuel line from the tank to the carb is clear. For some reason it appears to not be drawing fuel into the engine. The mower has worked great until this season. I replaced the spark plug as well and as far as I can tell there are no issues with the spark plug or magneto. Thoughts?

i would say its possible that carburetor is one with a rubber needle seat. ive replaced a bunch because they swell shut eventually. the kit number used to be 498260, i know its different now but they prob about 10 bucks. dig the old seat out w a pick or some will blow out w air pressure applied to the fuel inlet. push the new one with the groove facing away from you down into the seat hole gently but snugly into position with a 4mm or so allen wrench and reassemble. hope this helps.

Not sure exactly what the exact problem was, went through the carb again and also replaced the fuel line. It now fires right up and works fine as it always did before.