Check for spark cold, so you have a reference.
When the problem occurs, check for spark again and compare.

OR

Loosen gas cap because the vent is clogged?

Flywheel key

what is the flywheel key?

Here is a typical flywheel key replacement. When you remove the shroud on top of the motor, blow it out with compressed air. A dust accumulation around the fins will cause overheating and coil failure. When you remove the flywheel nut, you can usually see if the key is sheared.

Reprinted from a prior posting:

The customer drops off a lawnmower with a tag that says “The motor stopped and couldn’t restart”. I pull on the starter rope and I hear a scraping noise underneath. The first step is to see if the blade is bent.

{{gwi:131072}}

The blade hub comes off at the same time.

{{gwi:131073}}

Since the blade is bent, along with a broken hub, and the motor won’t start, the usual culprit is the flywheel key being sheared. Unplug the spark plug wire. If you have electric start, disconnect the battery. The top dust cover is removed first, then the gas tank is emptied and removed. The oil fill tube mounting screw is removed and the fill tub is turned to the side, but still in place. The shroud is usually held in place with 4 bolts and after they are removed, lift the shroud straight up. At this point, you’ll have the flywheel in view.

It’s obvious that the key is broken. Sometimes a motor will still run when the flywheel is this far out of alignment and sometimes a motor will not run when it is only slightly out of alignment.

{{gwi:131074}}

The flywheel puller tool (Briggs part number 19069) requires that two bolts be screwed into the flywheel. This is difficult without the use of a ¼” X 20 tap, and the flywheel is first tapped. You need at least 3 threads for the bolt to engage securely.

{{gwi:131075}}

Install the tool. There is a nut above and below the heavy steel bar. Turn the bolt into the flywheel until it bottoms out. It isn’t necessary to use the bottom nuts. Use a wrench to hold the bolt head and another wrench to turn the top nut down against the steel bar until it contacts. Do the same for the other bolt and nut. Alternate between the two nuts and turn the nut ¼ turn at a time until the flywheel pops off. When installing the new key, be careful that it doesn’t fall out of the shaft groove when placing the flywheel back on.

{{gwi:131076}}

You will need a suitable tool to hold the flywheel while you torque the center nut. I know that some will use a strong screwdriver to keep the flywheel stationary, but you might end up breaking one of the fins using that method. Sometimes the starter hub which goes between the nut and the flywheel has a tab on the bottom that engages a hole in the flywheel. If you torque to nut and the starter hub isn’t flat against it, the flywheel will break while being torqued. Always refer to a manual for the proper torque spec. Total job time: Less than ½ hour.

{{gwi:131077}}

I am skeptical that it is the flywheel key but I will look at it today. The mower always starts right up from cold and after it stops running it will start right up again after it has cooled down. I don't hear any type of scraping noise when starting the mower.

Great post from Baymee. With the shroud off and a 15/16" socket- you can easily tell if the key is sheared on these. If it is , see the above procedure. You won't hear scraping from a sheared key itself. You will get increased combustion temperatures from the change in timing. If the key was sheared- treat it to a new spark plug while you are shopping for the key.

Why would it take 15 minutes for the effects of a sheared flywheel key to become apparent and kill the motor? The motor warms up long before then. Would you rule out the ignition coil becoming overheated in that time frame? Are we assuming that the expansion of the metal parts of the motor have maxed out at that point? Wouldn't an engine running out of time produce black smoke on startup and run erratically? I have seen them do so.

Focusing on what we do know- the engine is running hot. I and many other techs (as well as someone in tune with the running of their mower) can sense a laboring of the engine plus the excessive heat. What happens at that 15 minute mark? It would really be conjecture. Myself, I always change a spark plug in an overheated engine. Fuel delivery issues could occur. A coil? You bet. A sudden change in operating and the heat gives you the clue to go there first. it is an easy check plus being concerned about the saw dust, one would pull the shroud - so you are over half way their anyways. On a Briggs or Tecumseh, you basically have to remove the nut to see. Maybe after all this the key is fine and he will need to look elsewhere. But it was easy and free...

Cylinder/head cooling fins blocked by sawdust, other chaff. It's heating up on you and seizing up.
Take the tank shroud and air shrouds off and clean it. This is an operation that is supposed to be done yearly anyway. If you had the sense to blow the dust out of it before using it you would never have had to post here. You can use a leaf blower for this, a shop vac with the hose on the vac exhaust or compressed air from a tank, not a duster can. That might still work and could be worth a try but my guess is it will not remove all the material from the fins.
Hopefully the numerous seizures it's already had will not now cause oil consumption from a scored cylinder.

My Sears 6.75 mower runs hotter than it should. Think the tube that holds the dipstick is loose and may leak.