12.5 horse Briggs and Stratton garden tractor will fire then sounds as if it isn't getting fuel. Any ideas?

Hi,
Often if it sounds as if it is not getting enough fuel, that is exactly it! Dirty fuel filter or carb are the most common culprits though a pinched fuel line or clogged fuel pikup in the tank are possible.
Of note is the fact that today's EPA-mandated engines and their emissions focuses caused some extreme changes in the ignitions to the extent that they contain electronics to change the timing. Nowadays an engine that sounds "down on power" may in fact have a partially sheared flywheel key. Years ago it would not run. Now it can often run badly.
Peace,
Clarke

Does it keep running if you keep the choke on?