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.

A good-sized air inlet and air outlet in your cowl doesn't guarantee good cooling. You have to make absolutely sure that the air entering the cowl actually passes THROUGH the fins of the cylinder and head of the engine. If you look into the air inlet and can see the air outlet, then the air's following that path and not cooling the engine. With larger models that have large cowls, you have to make baffles to make sure the air entering the cowl acutally passes throuth the engine's fins. The Great Planes Lancair ARF kit comes with baffling you fit to the cowl and engine to make sure the engine gets cooling.

When properly-baffled, you'd be surprised at how small an opening is needed. On a full-size RV-9A homebuilt, the cowl air inlets are about 3" x7" each (or slightly larger) to cool one-half of a 160-horse engine (two cylinders). It's making sure the air's put to work that's important.

THANKS FOR THE REPLY, I HAVE BEEN OUT OF TOWN SO JUST GOT TO READ IT AFTER TWO WEEKS. PLEASE READ THE ORIGIONAL POST. IS OVERHEATING THE PROBLEM ? COULD IT BE SOMETHING ELSE?
THANKS SCOTT B.

Yes, an engine losing power after a few minutes is usually a sign that it's overheating.

thanks for the reply, upon closer inspection I found that fuel was leaking from the back of the head! It looks bad so I sent into Hobby Services yesterday. IF you can do anything to get me a fast turn around it would be great!!
Thanks again Scott B