If an engine is overheating it loses power at the top end of the throttle. It normally smells pretty burned when you run it on the ground.

Maybe you need to turn it richer, both the low and the high speed needle.
Wy do you use so much nitro, they don't recommend it in the manual.
Check it on their website, [link=http://www.osengines.com/manuals/140rx-manual.pdf]1.40 RX manual[/link]

That prop seems a bit heavy, I would use something lighter.

Anyway, that's what I think of it but I'm not an expert. Maybe someone else can confirm this or advise something else.


Good luck