Shearing of flywheel pin on starting 12 hp snapper

Timing of the magneto to the coil is critical. so is the gap between the magnet and the coil. Double check both, the gap can be set to .01 to .019 depending on the make and model of the motor and you did not post that.

Take the cover off the housing that covers the flywheel. The crank shaft comes up the middle of the flywheel and has a slot in the end. The slot has the woodruff key in it. The key fits in the slot on the flywheel. When you hit something with the blades the crankshaft stops and the flywheel keeps turning. This shears off the woodruff key or makes it move out of position and then the timing becomes out of spec. The motor wont start. You need to pull the flywheel off and replace the woodruff key. They key looks like a half moon piece of metal.

I would assume the mower deck was engaged, check the shear half moon key in the shaft and flywheel. they are made to shear to prevent engine damage. other than that, check all the safty switches that keep the mower from starting unless they are engaged.

Indications that the motor is out of time: jerks the rope out of your hand when trying to start, pops out the carb, runs but has no power and sounds 'off'.
When assembling the motor a dot on the cam gear and a dot on the crank gear need to be lined up.
If the motor has run correctly before the problem and has not been disassembled then the problem is likely external.
There are no actual timing adjustments. The flywheel key holds the flywheel in the correct position. If it is sheared or partially sheared the timing will be off. Check the flywheel key by removing the flywheel and visually inspecting the flywheel key; replace if not 100%. On assembly be sure the flywheel nut is tight or it will shear the key (again), then set the armature air gap ( gap between ignition module and flywheel magnet) to .020
On older motors the points gap can affect timing also but I dont believe your model has points. The points gap is also .020

This happens often. Sometimes your mower will start and run again and sometimes it won't. When it doesn't run it's because your timing is off, caused by a dented or sheared key.
To replace remove top cover to engine, and any other side attachments, this should expose the flywheel. Remove the top bolt and look between flywheel and top of crankshaft, you will see a small square notch. This is where the shear key is placed. Remove the flywheel and replace the shear key. Replace all parts in reverse order and you should be mowing again. GOOD LUCK.

You probably have a sheared or partially sheared flywheel key. Replace the key if it indicates any damage at all. If the flywheel key is fine and the engine is over 5 HP then the internal compression release would not be functioning properly.

I hope this helps you out.

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.

Remove the blade and check the keyway. If the key way has the shear pin still in it it may have been bent and not sheared off. That will cause it to be off timing. I would imagine the valves got bent from the sudden stop of the engine as well

This usually means a bent or sheared key in the flywheel. Remove the shroud exposing the flywheel buy a 6" flywheel puller and remove the nut for the flywheel and attach the three jaw puller remove the flywheel and replace the flywheel key. The only other thing that causes this popping through the carb is a burnt intake valve this is a job for a lawn mower shop.

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.