Adjusting Injection Pump

[rion]

We have a Town Ace AT 3C desiel Engine. The Van is losing slight Power and seems late in shifting gears, excess Black smoke is begining to be more seen.

what our mechanic does is adjust the Injectors and the desired power is back to normal, Auto Shifting Gears are now on timing, smoke level is normal. but the problem slowly goes back again after 3 months of use. I'm wondering what is the cause of the problem, is there something must be done so the problem will stay away for good?

[rion]

the problem is fixed btw. the mechanic just fixed leaks and replaced Air Filter.now Van is running very smooth.(barely notice auto shiftings and doesn't demand for more RPM in high speeds)