Well, Mek-a-nik, I should have followed your advice and looked into the present Cub B&S engine for the problem. We began tonight by removing the spark plug, fuel lines and fuel pump, and blower housing. I then slowly turned the flywheel by hand until it stopped with a slight thud and I repeated it several times while by brother-in-law listened with a metal rod against the head and block. He surmised the problem was with the head portion. I couldn't sense the location as well, but I was still aimed toward the crankcase area. We decided to go ahead and remove the head after viewing the valve rocker area where the valve actions seemed OK. We removed the 8 head bolts and at first thought it looked OK, but then I noted the carbon on top of piston had been peened in a few places as well as on to the exhaust valve. Looking closer I noted what looked like a yellowish piece about 1/2" long, almost looking like a piece of wiring insulation. After removing it and another small 1/16" diameter piece, I removed the grime from them and found the larger piece was the end of the brass throttle shaft, since there was half of a threaded hole and the flat portion for the butterfly. Thank God it was brass and not steel! The piston had some light dings, and small slight deformity at the top OD that was easily corrected. No scoring had appeared to the cylinder wall.
What it now looks like it's a new carburetor or repair. The top portion of the throttle linkage and shaft wobble more than it should as does the bottom since there's no shaft there, so I assume it will be a new carburetor. I see in my Cub Cadet Parts Manual there are kits for everything but the throttle shaft I need and the carburetor body itself.
Now as I think back, I understand the symptoms leading to the consequent failure. The engine this past season would never slow-idle after working it mowing. You could slow it down, but then it might die if too slow idle so I had to keep it throttled up some. I assume the air seepage due to sloppy fit of throttle shaft was partly to blame here as well as burning ethenol fuel. Another symptom was after running, often times the engine would backfire when shutting down. I would idle it down hoping the engine heat would cool and not lead to the backfire, so I throttled it down to nearly dying and then turned the key to OFF. This is when it would sometimes backfire. This no doubt occurred sometimes with the intake valve open and the exhaust valve nearly if not entirely closed, putting this tremendous back pressure to the throttle butterfly and shaft, eventually causing it to weaken and break, causing the larger piece later to become ingested through the intake valve and into the cylinder. That is when after the engine had been hardly started, suddenly died, and when restarted, I then began hearing the knocking sound of the piece being hammered by the piston. I now understand why Cub Cadet says to "place the throttle control near the FAST position, then, turn the ignition key to the STOP position. Now much smarter and with a new 21 HP engine sitting waiting. Guess it can sit on standby until I can recoup my investment.
Anyway, I'm a bit smarter after being ignorant!! Now it's on to the carburetor tomorrow evening.