I have a 13.5 hp Briggs & Stratton riding mower, it lost almost power and backfires fiercely through the carb at any rev above an idle. I have already replace the head, flywheel key, cleaned out the carb and bypassed the fuel filter. It may have overheated due to a mouse nest found under the cowl. I have a new magneto/coil on order.
What can I do!! :mad:

Check The Valves.

Valves themselves are new with the new head, they are adjusted according to the manual.

What clearance do you have the valves set? I know you said according to the manual... but I have found a manual to wrong in the past.

The book says .004 to .006 , I think I used .005, is that correct?

Yes, The Reason For Using .005 Is To Stay Within The Tolerance Range .
.004 To .006 Is The Tolerance Range. Therefore, 005 Is Great!