Small Engines (Lawn Mowers, etc.): 17.5 Briggs OHV loss of power, ohv motor, head gasket

Question
I've got a Murray w/ a 17.5 B&S OHV motor.  31C707 Type 175E1.  Idles great.  Take it up to power and it starts to miss and has no power.  I've cleaned the carb, checked the flywheel key, replaced air filter, spark plug, fuel filter, lapped valves, continually checked valve clearances, replaced coil, cleaned out muffler, checked pushrods, replaced head gasket, and finally replaced head.  No help.  Am I missing something?  The mower is only a few years old and I'm losing patience.

Answer
The very first thing that comes to mind is either the governor is failing, or something was overlooked during the carb cleaning process. Are you 100% sure you reconnected the springs and linkage rods back in the right positions, in reference to the governor? Sometimes it is virtually impossible to get to all areas of the carburetor to properly clean it, hence the need of the ultrasonic parts cleaning tank we shops use. You mentioned cleaning the carb already, but a fuel circuit within may still be partially blocked. Did you just clean it or rebuild it with new parts or both? Was the float checked for gas inside it by shaking it close to your ear and listening for fuel trapped inside it? Was the needle and seat changed if it has a removable seat? I would seem to think it's a fuel system problem. These are the things I would check again.         Tom