Belarus 520 injector question

[belarus520]

We purchased a Belarus 520 from an older gentlemen. Looks like the head has been rebuilt before. After shredding our pasture we realized it was "gaining Oil". We swapped two of the injectors with new ones. First question: Some off the injectors seemed to be pretty clean at the bottom some seemed to have a crusty black buildup. What are they supposed to look like? Second question: The tractor has no miss, but smokes black and sometimes blue. If the oil gain is not from the injectors not working right what else could cause it? Also does anyone know of a person that works on Belarus tractors in Northeast Texas. Thank you.