RE: Ever had an engine that wouldn't lean out?
Bijan, the screw in the front of the carb is going to be an air bleed adjustment. If that is off, it can foul the way your engine runs all the way thru the range. But if you haven't messed with this setting, it's probably not that far out, and probably not the culprit.
I agree with others who've suggested new fuel. Old fuel can absorb moisture from the air and will cause problems just like you are describing. I have had fuel go bad once before in as little as 6 months. The 25% will not hurt your .40FP, so long as it has at least 17-18% oil in it. Ideally, you'd run 10-15% nitro in this engine, but 25% shouldn' t hurt it so long as you are running a prop in the 10x6 or smaller range.