I just pulled my old trainer out for my friend to start flying on. It has an Tower .40 ABC on it, and is fully exposed. I flew it about 5 times to get it all trimmed out and it flew great, no engine troubles at all. But now, the engine will start up just fine, takeoff, then after about 2 or 3 minutes, it loses power and has no throttle response from 1/2 to full throttle. I have adjusted the needle valve many times to try and get it better, but nothing seems to help. I am running 15% Byron Fuel in it, it ran Tower Hobbies fuel before I hung it up a couple of years ago. The engine is mostly clean, with a little bit of old burnt fuel on the head. Would overheating be the problem although it is fully exposed? Any help appreciated, thanks.