Sounds like the engine is running lean (not enough gasoline mixed with the air) even though you have the needle valves opened up. It may be the pump inside the carb is weak. If it is easy to do, here's a diagnostic you can try: Place the gasoline tank a couple feet above the engine and run the engine. If this improves the engine running, this indicates that the pump is weak. With a proper pump carb, you should be able to put the tank a few feet above the engine or a few feet below the engine and the engine should run the same.

It could also be a restriction in the fuel flow through the needle valve (e.g. some dirt is blocking the little passages that the fuel must flow through). Also, most carbs have a small scrren (filter) just inside the carb where the fuel nipple is. These can get clogged if the gasoline has some crud in it.