16/8 is the correct prop for this engine. How much time do you have running the engine? It may not be broken in yet. You should be running 35 to 1 gas/oil mix, start the engine then peak the rpm, then turn the needle valve out a little so it's slightly rich setting, run the engine for two minutes, stop it for about 5 minutes to cool. Repeat the run and cool 5~6 times, then try to fly it a partial throttle settings.

One thing you may want to check is the carb's location. If the carb is located where the wind blows perpendicular to the carb's intake it makes a vacuum and messes up the carb's metering. A way to test if this is the problem is to use a venturi stack cut at an angle, facing the wind, or do like a friend did to prove this was the problem which it was in his case, tape a piece of a coffee cup to the carb so that the wind couldn't blow across the venturi opening.