OK, so possiblity overproped from way back...
You report black smoke ar 2500 rpm
What rpm can you get with WOT.
It should be around 3400 to 3600 rpm.
Anything much less with a clean bottom and clean prop indicates a prop sizing problem if you have a clear air inlet and black smoke. Technically you could have a gear
box ratio issue but this is highly highly unlikely.
With enough air, the black smoke indictes too much fuel at the rpm set. Your engine is expecting to reach 3600 so at WOT, it is delivering enough fuel for that rpm. If external factors (prop size) prevent that rpm, black smoke is the result.
The air supply remains a constant, fuel supply is varied, not enough rpm gives a rich mixture, thus the black smoke.