Lawnmower makes loud explosive sound, black smoke pours from muffler and LM dies.

I have a Honda HRX 217. I had the lawnmower in storage for last season. I stored it after running the fuel tank empty. I only use ethanol free gasoline with added stabilizer.
This morning I took the mower out of storage. I filled the tank with ethanol free gasoline. The mower started on second pull. Seemed to run a little rough on idle but purred along well under full power as I mowed the back yard. About 30 minutes into operating the mower there was a tremendous bang, black smoke poured out of the muffler and the mower stopped dead. My wife ran out of the house. She thought someone had shot me! For 20 minutes I tried unsuccessfully to restart the mower. Then I happened to notice that the spark plug connector was disconnected. The explosion had blown the spark plug connector wire off the plug. I reattached the spark plug and the lawn mower started up on the first pull. Again it seemed to run a little rough on idle but seemed fine under full power. I mowed part of the front of my yard for about 10 minutes and then the whole thing happened again just as before with a powerful explosive sound and black smoke from the muffler, disconnected spark plug.

Any ideas what is causing this? I'm worried that it might be dangerous to continue to operate this LM.
John