I'm not very familiar with that engine but i do know diesels ok. You said you had black smoke so lets start with that before pulling anything apart.
Black smoke indicates that there is either to much fuel or not enough air.
* If there is an air cleaner check it and replace or wash if needed.
* If the air cleaner or foam filter is OK it could be a restriction that presents under a load. See if you stall or lose power with a NO load rpm
test. not a bad idea to check the exhaust
temp with a pyrometer if you have one.
*It could also be a bad injector. The injector needs to atomize the fuel to get your power, clean burn and efficiency. Especially in a single
* an easy little thing I would recommend too would be to use Lucas or Diesel Clean fuel additives. (They sell it at walmart or any auto parts
store) it makes a big difference lubricating and keeping you injectors clean. I barely get any exhaust soot on my transom anymore.
Good luck, hope its an easy fix.