I just built the plane you are. The kit directions say not to run more then a 6 screw prop! The carb being below the tank has not caused any problems with my Saito FA72. I run 15% omega and it seems to lilke it fine. I run a 14x6 Master air screw " K " prop and it is awesome. I ran mine rich, on the ground, for two tanks then leaned it out just enough to hold an idle and run then took to the sky. With the 72 and this prop, take off roll out is about ten feet then vertical at 1/2 throttle. Keep the CG to the front for a bit this plane realy is Dr. jekyll and Mr. hyde.