I have a Briggs & Stratton Twin cylinder 16 hp engine used by a Simplicity lawn tractor. So far, I have installed a new battery, new spark (and gaped) plugs, new air filter, new fuel filter, and changed the oil and filter. The flywheel has been removed and the keyway has been inspected. It is intact and not sheared or damaged. The engine will start with the air filter removed. It constantly backfires through the carburator, along with a metallic pinging sound from the engine. What would be my next course of action?

Check your connecting rods... metalic knocking is NOT a good sign... once something breaks and ventilates the engine block.. it goes from repairable to scrap metal.

Hi,
Chaecki the valves. Will give links if uo give numbers.
Peace,
Clarke