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.

Most likely old crud in your fuel tank.

Enjoy,

Jim

If there are no leaks and that's a big IF, it just sounds like the mixture is too lean and yes that will cause overheating. Richen both the high speed needle and low speed needle about half a turn and fly again. If this cures the problem then reset both needles to their optimum slightly rich settings and you should be all set. If the above doesn't work the you need to clean the engine and fuel system and check for leaks or blockages everywhere. When an engine and plane have not been used in a long time things get gummed up, fuel lines crack and gaskets get dry and start to leak.

Denis