My first post here in long time, all kind of excuses mainly old age medical problems.

Sears lawn tractor not sure age but several years old
Mod.917.254791 Ser. 13615248351
12.5 HP OHV 143 416082 SER 1130D (the D could be 0)

After mowing about thirty minutes engine begin to slow down as if overloaded, then began to back fire occasionally.
My first thought water in gas, drained and cleaned tank also removed carburetor cleaned used air to clean. There is a inline filter but there was small amount of trash in the bold.
Back together starts and runs great as long as there is no load, Engage Mower it starts same pattern again.
Then I check valve clearance Intake about .06 Ex. .010 nuts all tight so I guess these are OK, don’t have any information on this engine.
Next check governor, linkage seems to need adjustment after attempting adjustment no improvement.
Next I remove linkage run engine at various speeds the governor arm moves some but not with enough force . So now I think it is probably within the crankcase.
Any comments before I go any further on this?

Could be a leaking head gasket, air leak to intake manifold, plugged vent in fuel cap and or possibly a faulty ignition module, but I doubt that it's the governor.

could the engine be getting too hot due to debris all over the cooling fins and surrounding areas?

That is another possibility...

Thanks for the suggestions, I will check and get back with results.

Checked Head bolts all were tight, pressure test 70 lbs. the engine is clean. Did not find any problem with air leaks in intake manifold all gaskets good and blots tight.
Air gap at ignition module set at .026 ( is this correct? )
Is there some way to test the ignition module ?
Vent in fuel cap is clean.
I have not replaced spark plug but I guess that could be another possibility
Forgot to mention I did disconnect wires that control all safety switches, and use jumper cables to start.
IT starts real good every time, does require choke but this has always been requires.

http://www.cpdonline.com/695244a.pdf
Here is a link for a service manual that should cover your engine. I don't think that is the proper setting for your mag air gap, but you should be able to find the correct setting in the manual.

most generally the air gap is .010-.015 I do believe

.20 is normally for the points and condensor

Had problem with the service manual download after almost three hours I gave up.
I set air gap at .016 this did improve some then I set at .012 big improvement I was able to mow about 15 minutes then my original problem occurred, first thing I turned flywheel to see if it turned freely and it did.
Next I will try .010 but beginning to think the ignition module may fail after it gets hot.
Reminds me years ago had a coil do this on a twin cylinder engine.