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Could be fuel, spark or manifold leak. Take a good look at your spark when it's cold and again after it warms up and fails. If you still have a good spark after it fails, move your attention to fuel delivery. Get a can of carb cleaner and as it starts missing, spray it around the manifold gasket and carb to manifold gasket. If you have a manifold leak, you'll hear a difference as the carb cleaner gets pulled into combustion. You also can use your choke as it's starting to run rough. If the problem is fuel delivery, additional choke will appear to fix the problem.... at first. Make sure your engine is leaning out rather than choking out. Sooty plugs tells of too much fuel and that can do the same. Plugged air cleaner can do that and your problem would resolve if you remove the short air hose from the cleaner to carb. (Don't run it that way but it's a good elimination test.) Your problem is still simple. Keep thinking "Air, Fuel, Compression, Ignition." One of those is changing after warmup. Have you looked at compression? It's so easy to "think around" possibilities but when things don't make sense, you have to go back to the basic 4 and check each one. Good luck.
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