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Is there a FLOAT DROP setting on your carb? That is an extra tab on the float that controls how far the float drops when the float bowl gets very low of gasoline. Lots of carbs have this setting, but I haven't been lucky enough to OH an 'N' carb for several years, and I don't recall if they have a float drop setting. If it has a float drop setting, this should be set to spec, or if there is no float drop setting, a bent float could be hitting something in the carb, preventing the float from dropping enough to open the needle valve enough to allow gas to flow into the carb fast enough to keep up with engine demand. Have you tried a different needle and seat? Sometimes the needle and seat assembly used in a type of carburetor will vary with the application... size of engine, or whether or not a fuel pump is used. Could the wrong one have sneaked into there? With a cool engine, not running, when you remove the drain plug from the bottom of the carburetor, you will get a fast flow of gas, for the first half-cup, or so that the float bowl holds. Then the flow will slow down to the amount that can get into the carburetor through the needle and seat, and as long as this flow is somewhat more than the engine needs at full load, it is adequate. I'm sure you've checked for a good flow of gas from the gas line, at the carburetor, and checked your screen that protrudes into the carb from the brass elbow gas line fitting. Check the passage that the screen/elbow screws into for foreign material blocking the flow of gas to the needle and seat. If you are sure the problem is related to a shortage of gas, try spraying a stream of carburetor cleaner into the carb air intake while it is sputtering. If the mixture is lean, the carb cleaner will richen it and settle down the motor. (Use good old-fashioned carb cleaner such as Gumout for the test. Modern enviro-friendly stuff probably won't "burn" as well in the motor.
If you decide to do this (you'll have the tube from the air cleaner removed), have a fire extinguisher handy, and keep face and body back, 'cause if it bacfires with the air intake tube off, it may puff out flaming gasoline. If problem is definitely not ignition, a couple of other freak things... Bad head gasket causing slight combustion gas leak between two cylinders, or an intake leak... hidden crack in intake manifold, or bad intake gasket that leaks worse as engine heats up.
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