It could be a dirty prop or over propped situation but if cleaning
the prop doesn't fix the problem have a diesel
shop clean and calibrate your injector(s). White smoke is generally unburnt fuel
in the combustion gases. Black smoke is generally when your engine
is overloaded. However because you can't get anywhere near your high rpm
when in gear it could just be a dirty prop. If you put your hand in front of the exhaust
outlet when running hard does it smell like diesel fuel
afterwards? That is one indication of unburned fuel.