What is wrong with Diesel transit non turbo van which has lost power but if revved really hard it seems to clear?

hey, get something called *fuel injector cleaner* im guessing its got a carby. so get your fuel injector cleaner possibly 2 bottles (around 5 bux a bottle) start car. get someone to sit inside and rev it from 1000rpm - anywhere high. when the revvs go up. pour in some injector cleaner. you will get LOTS of smoke from your exhaust. this is normal. to get to your carby. follow your air cleaner to the engine that is your carby. after that let sit idle for a 2 mins. and drive away. 90 % of the time this will fix the problem.