WHAT WOULD CAUSE THE FLYWHEEL KEY TO BREAK SHEAR OFF?IT HAS HAPPENED TWICE NOW.THE PLASTIC GUARD LOOKS FINE.

The kill switch may be mis adjusted. Sometimes the cable stretches and you need to adjust it or shorten the cable.

Hi Duane:
I'd be thinking that it's possible that the alignment key on the flywheel has sheared. That will throw the timing out and it can't run.
Fix is pretty straightforward.
Disconnect and remove pieces until you have the flywheel exposed. Loosen the nut. Lift on the flywheel and smack down on the nut with a plastic mallett to loosen the flywheel. Then remove the nut and flywheel. Replace the key (get replacement ant any lawnmower or chainsaw service shop)
Put it back together and it should run just fine.
Cheers.

A back fire will cause a key to shear. Have you set the valves yeat

you will have to check flywheel to starter drive clearance if this is off then it can sheer teeth off. also check the flywheel teeth as they maybe damaged in which case the flywheel will have to be removed.

Did you hit something with it while you were mowing? Push mowers have a aluminum key in the flywheel that will shear off if you hit something with the blade. Sometimes the crankshaft will get bent when that happens, but almost always the dey will shear off. This will cause the spark timing to be off and it will not start. You will need to take the pull starter off and the flywheel nut. You should have a flywheel puller to pull the flywheel off, it is on a tapered shaft. You will find an aluminum key that is sheared off and you can get one at any lawn mower repair shop. The only other thing that would explain gas on the plug and you have spark but it still will not start is the gas being old and will no longer fire (as long as you have compression in the cylinder). Please rate my answer so I get points for it.

Did you hit something with your mower? It sounds like a timing problem. The shear key may be dented throughing off your timing. The shear key is located between the flywheel and the top of the crankshaft. Remove the top cover to your engine and any attachments. Remove the flywheel. Check the key, I believe you will find your problem, a deformed shear key. Get a new one, replace all parts,and you should be ready to go. GOOD LUCK.

Did you hit something with your mower? Check to see if your blade is loose. If so tighten your blade, if not then your problem is a timing problem. Remove top cover to your engine and any attachments. You will see the flywheel. Between the flywheel and the crankshaft top is where the shear key is located. Remove the flywheel and inspect the shear key. If it's dented or completely broken replace. Put all back together and your mowing again. GOOD LUCK.

check the valves. if the flywheel bolt is tight it shouldn't shear the key. i wouldn't run it with out the covers on. it will over heat and make the exhaust glow as you said.

You need to remove the nut then hold the flywheel in your hand and hit the end of the crank with a plastic hammer the flywheel is a tapered fit with a key , Just remember the new one has to be put on tight or it will shear the key off.

Found that I had left the comprssion washer (flywheel washer) off when I assembled it the first time. Put it on and torqued to spec it runs fine now.