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Vintage Sears Garden Tractors

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Vintage Sears Garden Tractors
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B&S type 19 engine 7.25 hp cast iron block: gets hot backfires, shuts down. starts up when cold.

when the engine is running at full throttle, and getting hotter and hotter, it begins to backfire, and when the throttle is lowered the engine runs better. but as it really gets hot, it shuts down, even at idle. I put new points and condenser in, went through and rebuilt the carbureator and a new float. put new gas lines on and cleaned the gas tank out. installed new spark plug. I checked the valve springs and valve clearance, intake and exhaust, all in specs. moved cylinder head, cleaned carbon, and checked for seating. I've noticed on the intake stroke, the intake valve lifts up before the exhaust valve seats. Is this normal, or a quick release design. If so what can I do about this? maybe this is my problem? should I give it more valve clearance, or is the cam shaft at fault. the flywheel cools the engine down, no obstuction there, but still shuts down when hot. i tried running the engine straight pipe, but same thing happens when hot. I'm puzzled.