I have gp extra 300 74" with 160 fx. set up exactly the same as in the review here on rcu. I do have more exit air than the pics shown, about 2 sq inches at the engine head for glow access and bigger cut out in the bottom of cowl. almost twice the size open on bottom.

Problem is that after about three min. flying power starts to go way down two more min. and I'm doing dead stick. the smoke trail stays good so I do not think it is getting lean.