Sounds like you are too lean on the high end, even with the NV opened up. This can be caused by constricted fuel flow somewhere in the system. Check the easy things first. Unhook the line at the carburetor and connect a few inches of tubing. hold this tubing below the level of the tank and make sure that the fuel siphons freely from the tank. I have had similar symptoms when the carb filter is plugged up. there is a mesh screen inside the Walbro carb right where the fuel flows in. This can get clogged with a layer of lint-like crud. Also the pump diaphragm or regulator diaphragm can get weak also resulting in less than enough fuel flow. I know you have already tried two carbs, but perhaps each of them has a problem. The temp of 300-325 seems too high, also a symptom of too lean. Finally, air leaks in the case or around the carb block can let air in also causing a lean run. Air leaks are usually visible upon close visual inspection.