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?