Small Engines (Lawn Mowers, etc.): Loss of power, briggs stratton, valve damage

Question
I have a Briggs & Stratton 14.5 HP on my mower. The carburetor was professionally rebuilt. I replaced the head due to bent push rod and valve damage. The new head came with valves. I adjusted the valves at .004 intake and .006 exhaust. This was done moving from TDC 1/4 inch down on the piston. The engine starts and runs with a slight miss but when the mower blades are engaged the engine dies. Any help is appreciated.

Answer
Is it electrical?  The PTO is usually tied into the ignition system with an operator presents switch.  Like a seat switch and a brake/clutch switch.  The PTO also serves two purposes one to engage the PTO and the other safety system.  Both have to work.  The miss could also be electrical or recheck the valve settings.