We normally prop the engine for mid- to high-7,000's. Any prop that turns that RPM on the ground and flies the model well is the "correct" propeller. We normally run a 16 x 8 to an 18 x 6 prop. The 18-inch prop is just about the largest you can use. The fuel you're using is OK.

One thing you didn't mention was whether you increased the idle RPM and then reset the carb. The engine can idle as low as 1,800-2,000 RPM, but that's much too low for good overall throttle handling. We usually set idle RPM around 2,200-2,300 RPM.