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I have a troy bilt self propelled walk behind mower with a Briggs & Stratton 6 hp engine. This engine has the by now infamous plastic carb and it wants to stall out after each 15-20 minutes of use.

I can pull the air filter off and choke it using a finger to keep it running. It misfires like crazy, Sometimes it will smooth back out and I can use it for another few minutes, sometimes it just will not keep running.

I need some advice on what could be the cause of this unusual behavior and what to do to stop it. The mower is 2 years old, the carb drained and fresh gas filled.

I am stumped. Help!

Clean the jets again or replace it, $20 or less.

By placing your finger over the air intake you're lessening the amount of air entering the carburetor/intake. That means you're running lean and not getting enough fuel, or you have air being sucked in around the intake.
give us the model number of the engine so we can see what the setup is. should be stamped above the spark plug or muffler.

If it is the plastic carb i think it is i had one i replaced the carb 3 times to find one that worked. Briggs & Scrapem's finest work!

Sounds like starving for fuel. Blow out fuel line, replace filter, investigate to see if air leaking in at intake. If carb is only $20 good idea to have at lease one new one on hand. I've had to replace carbs in 3 of my chain saws. Usually about $15 or so.

Good way to check for air leaks around carb is WD-40 in a trigger spray, no aresol.
spray it around the carb, and where any intake gasket is, if the engine speed changes or it may blow white smoke, you've got a vacuum leak.

Check your gas tank. I have a Craftsman with the B&S engine with the same problem. The new gas tanks have coating on them that peels off. Look inside the gas tank...