N1EDM, think you for your response.
Interesting. I found out today my friend was running the same engine with a 14 x 8 also, and had not problems. Now, here is my problem: A further review of the installation, showed that the tank position is above (way avobe) of what OS suggests. Way high. Bad news, is that because this is an ARF (phoenix PC-9, .91) I cannot move neither the engine nor the tank.
Therefore, from now on, I follow this new issue in
http://www.rcuniverse.com/forum/show...5&goto=newpost
given that the problem moves from the propeller to the tank location. (And I am going to the 14 x 8 without arguments, to start with)