I have a Cat 215 B excavator with the 3204 engine.  It ran great this spring, but has been sitting all summer.  I fired it up and it seems to run good until you put it under load.  Then it starts making black smoke and loses rpms fast until you stop whatever you are doing.  I changed both fuel filters and both air filters.  I checked the air inlet for obstructions and found no problems there.  Exhaust pipe appears clear too, but I didn't tear it apart (muffler has been removed and it has a straight pipe).  Fuel flow is good at the filters.

I am thinking that it may have a bad injector nozzle (or nozzles).  The filters were pretty black with algae.  Will the algae plug a nozzle?  What is the best way to figure out which nozzle is the culprit?  I don't have one of those infrared guns :(

Or is there something else I should look at first?  Thanks in advance!