I have a Yard Machine riding mower with a Tecumsah 13 HP engine. The mower starts fine and runs fine but after about 30 minutes when the mower is hot the mower begins to stall. Disengaging the blade and the stopping the mower is the only way to keep it running. At this point, even moving the mower with blade engaged up the slightest slope causes the engine to sputter and run poorly. I have to let the mower sit for a few hours before I can run the engine and cut with it again. At that point it will cut but may not cut as long as it did during the earlier run.

DG and Crigby were helping my neighbor with similar symptoms yesterday. On that Kawasaki engine we suspect the ignitor or ignition coil. While we wait for Clarke and DG to stop by maybe you can get us the engine model and serial #

Could be the coil / amuture trying to shut down,
But I would also look at a possible vapor lock on the fuel system , chewck and make sure the fuel cap on the tank is venting.
Luck

I'll have to get you the model # and serial # later today when I'm at home. For now I will say that I have suspected the coil. I don't think there's an issue with fuel cap. It has a hole in top of it and seems unobstructed. I open the cap and fuel up on most days but will make sure. It hasn't been that in the past. Next time for me to mow is coming up in a few more days. It probably would be good to know how to test if coil is defective when get to that point?

Get a can of spray carb cleaner , when it dies remove breather and spray in carb, if it starts fuel problem , if not probably no fire / coil .
But I see its not dying
Will it start right back up if you turn it off ?

The model number is OV358EA on the engine. There is no serial number indicated as such, on the label for the motor. The model on Tractor is 13A1762F029 and the Ser. # is 1D168B40227.

Again, the engine always restarts after the stalling problem occurs... but the problem has more to do with the sputtering and poor performance while the engine is under any kind of load after that point.

Sounds like you could be having a valve problem ,
Need to check your valve clearence.
Check it cold

If your diagnosis is correct, it would appear I would need some level of understanding for what an appropriate valve clearance in order to fix this myself. Correct? Is this a repair a novice repair man can make? Or am I making this sound harder that it should be. Where can I go for more info on what the valve settings should be and how to adjust them? Any recommendations? Thanks again for all your help... it's really appreciated!