I rebuilt the carb and checked to make sure that it was getting gas flow, I have checked and rechecked carb for dirt, etc.. Could there be carbon build up inside the mower keeping it from starting

Have you checked the spark plug for a nice powerful blue spark? If you unscrew it and put the wire cap back on then place the threads on the metal case, you can try starting and examine the spark. A yellow spark is too weak for proper ignition and could mean a bad spark coil. Watch it, if you hold it with your hand you can get a real nice jolt.

Good advice so far, does it poop at all if you spray a bit of starting fluid into the carb?