When you say the carb is open 2/3, do you mean the throttle stick on the transmitter or are you visually confirming that the carb is open 2/3?

How do you have the engine mounted? How far from the fuel tank? What muffler are you using?

You might be losing too much pressure in the fuel tank if the exhaust pressure line is too long or has a hole. If you're not using the stock muffler, or have a long header installed, this can also affect fuel tank pressure.

You may have an obstruction in the fuel path thats preventing additional fuel flow. I'd take the carb off, break it down and clean and back flush it.

At full throttle, is the needle valve effective? How far do you have to turn it to get it to reduce RPM? Are you certain you're running it to the rich side?

The problem you describe at idle could also be due to a blockage.

Brad