!!!! THIS !!!!
Oil burners are NOT SIMPLE, even thought they "seem" to be. There are many, MANY seemingly simple conditions that can ruin your day
When an oil exchanger soots it is a rolling snowball. Or if you prefer, fireball. The worse it gets, the worse it gets.
Many things can cause sooting, including an inept service person. YOU NEED a good service outfit WITH proper instrumentation. A draft gauge to insure proper draft, stack temp gauge (Nowadays likely included in the electronic combustion analyzer) and someone who understands how to LOOK at, SERVICE, and troubleshoot a burner. In my day, some of the people wandering, stumbling around town with a pair of channel loks, didn't even know that most pumps have a filter IN THE PUMP.
You have GOT to get yourself a COMPETENT service guy and get him out there at least every two years
