In my experiences a 11x6 prop may be too big for the engine. I run a 10x6 prop on them Ok.
You might be loading it down too much.
The engines are not a powerhouse. They were designed as a entry level engine that is easy to use and run.
if the prop is too large, it loads the engine down a lot, and the engine doesn't draw fuel at the lower RPMs very well.

Did you hook up the muffler and run the fuel tank vent tube to the vent fitting on the muffler?
 A little muffler pressure is good for the fuel draw.
Sometimes the fitting has the hole in it blocked, it's worth it to poke a paper clip end or something down it to see if it is blocked or not and clear it out.
Also try to keep the fuel lines going to the carb as short as possible.
You don't see any air bubbles in the fuel lines do you? Especially the short fuel line from the remote needle to the carb.
What size fuel tubing are you using? Small, medium, large or extra large. I think medium is the one to use on the engine.