Coal burning and oil burning steam locomotives generate smoke for entirely different reasons. With a coal burner, some smoke is a constant. Whenever you add fuel to the fire, it takes a few seconds for it to ignite, which results in thicker smoke. A coal burner is also throwing cinders as it exhausts, which also darkens the smoke. When an oil burner smokes, it is usually the result of over firing. Sometimes this is for show, sometimes because there are factors effecting the firing, and sometimes its just poor firing.
On an oil burner, the person firing it must stay right with the engineer's use of the throttle. More throttle, more fire, its that simple. If they are slow to respond to changes in the throttle, they will see the boiler pressure drop rapidly. When starting from a dead stop, as the throttle is opened, its best to open the firng valve a little too much, and let the increased use of the throttle clear up the stack. Otherwise you can be robbed of 10 or more pounds of pressure just like that. As you roll down the tracks, you must watch the engineer closely, increasing or decreasing the firing rate to match their use of the throttle. You want to maintain a very slight haze at the stack, while keeping the boiler pressure as consistently as you can to within 2 to 3 pounds of the pressure the safetys are set at without lifting them. But, as the "prime directive" is to make steam, if overfiring and smoke are neccessary to maintain pressure, you live with the smoke.
Burning oil causes soot build-up on the walls of the tubes and flues, reducing the transfer of heat to the water and making it is neccessary to "sand" them. While the locomotive is being worked hard enough to create a very strong draught, a couple of scoops of sand are fed through the firebox door peep hole. The draught literally sucks the sand out of the scoop and down the flues, exhausting out the stack and cleaning the flues in the process. Needless to say, this can create some impressive smoke plumes.
Besides mechanical and/or fuel issues, some locomotives have characteristics thet may result in the need for over-firng and black smoke. The 4449, normally a very easy locomotive to fire, has what we jokingly refer to as the "fireman's black hole". When the engineer is using between 45 and 60 pounds of cylinder pressure, it uses more steam than can the blower can create enough draught to replace, but at that pressure she doesn't have a strong enough exhaust draught to replace it either. This means you must over-fire, creating lots of smoke, and use every trick you know to fight the certainty of losing pressure. Doyle will use ever trick he knows to help you, but sometimes the only choice is to make a brake pipe reduction and literally drag the train a little to work the Daylight harder.
Then there was the time at Roaring Camp and Big Trees in Felton, CA. The fuel supplier accidentally dumped more than 100 gallons of acetone into the ex-Westside Lumber Company Heisler number 3's fuel tank instead of oil. Even though we stirred the oil and acetone with air, for the next two days, even with the firing valve all but closed, we had a brilliant white flame and the safety's were lifting most of the time. An inspection of the firebox and boiler tubes revealed them to be absolutely clean and free of any soot what so ever, LOL.
Tom