Is it a Imperial 300? Or is a imperial 300 50/70 (it will be marked in the top cover) If its a plain ole 300 it isnt really big enough for that 390. But either way ther is a load screw in the side of the elbow try it full rich and see if that helps. Also make sure you do not have any air leaks like a missing gasket between the carb and adaptor or a cracked hose