Oil burning furnaces need maintenance, more than gas or propane burning furnaces need. My own experience says you can go as long as 3 years without servicing it, but that is not advisable because sometimes the failure that results is severe (electrode insulator cracks, and you get the Bessemer Process on the electrode mount, which is to say, it melts). You are far better off cleaning them annually.
Cleaning and servicing them is no big deal, but it is messy since that soot is tarry and stinky.
Turn off power to the system. Remove the oil burner assembly, which will also have the electrodes on it. How you remove this assembly is very dependent on the particular unit; follow the oil line out of the pump to where it goes into the furnace housing and that will give you a good clue. Often (usually) you have to either remove or hinge back the high voltage transformer to get at this assembly.
When you have the burner out, remove the nozzle and replace it. Make sure you replace it with a nozzle with the same spray pattern, though you can vary the gallons per hour (GPH) rating to rate the furnace up or down in capacity.
Spray patterns are described as either solid, semisolid, or hollow – which refers to the distribution of oil in the spray cone, and have a specific angle which defines the angle of the cone of oil that is shot into the furnace. Solid is the most common pattern in my experience and 85 degrees is the most common angle that I have seen.
Wipe the whole assembly down to get the oil and soot off of it. You will need to position and gap the electrodes, but first do a visual inspection of them. If there are visible cracks in the insulators, replace them. If the ends are eroded and pitted, replace them. If in doubt, replace them.
There is a special tool you need to position and gap the electrodes; do not try to do this without that tool. It only costs about $4 at a supply house; I repeat DO NOT TRY TO DO IT WITHOUT THE TOOL!!!! You can have a fire, and in the worst possible case, an explosion that wrecks the furnace if you gap them badly enough. Make sure you gap and position the electrodes to conform to the particular spray pattern of the nozzle.
When you have done all this, take a rag and wipe down the air passages leading into the furnace. Wipe off the squirrel cage on the combustion blower. Wipe off the oil pump, the transformer, and the electrodes on the transformer. Also look for an optical flame sensor (some units have them) and wipe it clean.
Reassemble the furnace and fire it off. The flame should start instantly and quietly – no WHOOMP, no delays followed by a WHOOMP. If it is adjusted right, it will just start, immediately and quietly.
You then need to adjust the air to the flame. Open the inspection port so you can see the flame. Turn down the air until the flame becomes dull yellow and smoky, then increase the air until the smoke just vanishes and the flame “flower” is bright yellow. If the flame “flower” becomes too lean, it will be very bright and tinged with blue. At this point, you are sending too much air in and sending too much heat up the flue. When the inspection port is closed, if the flame roars and the furnace kind of “shudders” a bit, back off the air until that stops, then look again in the inspection port to make sure the flame is not smoky.
When all this is done, allow the furnace to reach operating temperature, then adjust the flue damper. The flue damper should swing freely and not catch, either open or close. If you cannot achieve this, replace it. When you have achieved this, adjust the damper so that it is swinging about half open when the furnace is running and at temp. When the furnace stops and the flue cools, the damper should swing shut on its own.
Beyond this, oil the pump motor, oil the blower motor, change the filters, tighten the belts, replace the oil filter, and you are done. You could also test pump pressure but that does not change unless either tampered with or if the pump is failing; I almost always ignore it.
Although this sounds like a lot, when you get good at it you will do it end to end in 1/2 hour. You will also stink of fuel oil when you are done, so wear old clothes.
Oh yes. Must not forget. You WILL spill a certain small amount of fuel oil over the course of this; have some kitty litter (oil dry) available.