Small Engines (Lawn Mowers, etc.): 21.5 hp B&S sputters backfires loss of power, gasket leaks, air filter oil

Question
I am running out of things to try so I hope you can help.  I have a craftsman rider model 417.272450 with a Briggs 21.5 hp motor and I am having a backfire and power problem.  It starts and runs fine on low and mid idle but when I run it up to high for mowing, it sputters like it wants to stall and backfires and only seems to have about 1/2 of the power it should.  I have replaced the fuel, fuel pump, the magnetos, the plugs, air filter, oil and filter, fuel filter (twice), pulled the bowl off the carb and cleaned it out, and nothing seems to help.  The carb isn't adjustable and it just started all of a sudden one day with no major issues to cause it.  I have stumped a few of the small engine repair guys around here and don't want to spend a fortune on a repair guy that will do what I have already done.  Thank you.

Answer
Remove the carb, just taking off the bowl won't do it.  Discard the gaskets.  Disassemble the carburetor.  Make sure you get the main nozzle out too.  Use a wire from a twist tie to clean all fuel and air passages.  Blow it out with compressed or canned air.  Purchase a new carburetor kit and mounting gaskets.  Inspect the throttle and choke shafts for wear and replace if necessary.  Install parts and new kit.  reassemble the carburetor.  Check the intake manifold for cracks and gasket leaks. re-install the carburetor and attach the throttle and choke cable.  Adjust the cable.  This should fix it.  If not get back to me.