Black smoke going up hills

My LH119 (3L diesel engine) puffs out quite a lot of black smoke when going uphill, and sometimes, when first started from cold.  I'm getting about 25 MPG and the performance is generally okay, can get up to 70mph and above on motorways without difficulty, although going uphill does slow me down.  Should I be worried about the black smoke on hills? When it happens, easing off on the throttle cuts down the smoke a lot (although I end up going uphill even slower).  Any suggestions?  Nick

How's the air filter? Any other possible obstructions in the air intake? If not you may need to back down the fuel adjustment on the injector pump very slightly as it sounds that you are slightly over-fuelling.