Had a
Perkins 6-354 for many years. It started smoking,fuel turned very dark. Ran with clean fuel from a portable can & smoking stopped. Took injection pump to a shop & had it rebuilt...end of problem with that motor. I now have a 4-108 in a different
boat. Last year fuel started looking very dark & I thought it might be the same problem,but had no smoking issues. I soon discovered that it was sludge in the tank that appearently started breaking loose & mixing with fuel due to running in much rougher seas than the boat had been in for years.
Tanks were old so I replaced them. End of that problem. So..............if motor is smoking abnormally try running clean fuel from a another source. If smoking stops go with 1st senario. If motor is not smoking abnormally check
fuel tank. Hope this helps.