Cranky ; Not trying to split hairs , yes "no ping or carbon knock involved" , I'am in agreement with your potential for a glowing ember (carbon based) causing ignition of fuel within the cylinder prior to the spark plug firing sequence (BTDC) . In the Automotive Field Field it is referred to as Pre-ignition in severe cases , with Valves closed or still partially open , Not Rocket Science Bro , is there another Synonym you would prefer ? In either case it was not established in this Op's situation or Denny's for that matter , nor was a dirty air filter , although it may be suggested since Denny has advised that his spark plug was over 13 yrs old . Most prudent individuals clean or replace air filters when warranted depending on air quality conditions. However , dirty air filter is not the only situation for backfiring lean fuel mixture under load will also . As I said numerous items to consider lol . Anyhow Good discussion , I believe we are on the same page ! P.S. What are your background credentials , former Diesel Mechanic and 3rd Class Stationary Engineer here , have not seen your username on this Forum previously ?

Ok so its not a faulty carb flooding the engine. It seems more that the excess fuel is getting naturally aspirated into the cylinder but not combusting. Then exhausted to the muffler where it eventually does with a bang. I'm leaning towards a weak or intermittent spark at this point. Can a coil go bad where it delivers a weak spark? Can the resistance of the coil and/or plug wire be checked?

I really thought it was going to be the flywheel key. I checked the key, even took off the flywheel to make sure and it was perfect...it had not been damaged at all.

My Gigi plant was attacked by spider mights when I first got it. Someone on this forum (maybe Seil) said to wash the leaves including the undersides with a strong spray of water at least 5 days in a row to break the spider mites life cycle. I did that and it worked. I have continued to wash the leaves of my miniature plants at least once a week, sometimes more in summer. They haven't had a problem since, knock on wood. My miniature is in the ground, so I didn't have the option to immerse the plant.