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Gas engines need fuel (fuel/air), spark, and compression... If the spark and compression are fine, go to fuel/air. Look at things that may restrict gas flow. Since you've already taken care of the carbs I'd suggest looking at other parts of the fuel system. The petcock if rusted out can restrict gas flow or cut it off completely, a severely dirty fuel filter can also restrict gas flow. The gas itself if old can also cause problems.
If the petcock, fuel filter, gas lines, and the gas itself is fine, then i'd say it's on the lean mixture side. If not that, then dirty air filter restricting air flow.
The valves will also play a large part as if they're severely out of adjustment it will cause running problems.
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