If there is no Aquahot certified tech in your area, and you aren't intersted in learning about the exciting field of oil burner maintenance, I'd look around town for somebody who services oil-fired equipment such as furnaces. A furnace oil dealer should be able to point you to one. Mechanically the burner system is pretty simple, just a pump, a filter, a valve or two and a nozzle. The electrical side is more complex as it has defined sequences of starting and stopping the fan, the oil, and the spark for safety.
But normally a sudden change where it still burns means that a bit of dirt has clogged the orifices in the nozzle disrupting the pattern of oil spray, so that you're not getting full combustion. Usually a nozzle replacement and vacuuming out the combustion chamber will solve that proble. But the first thing I'd check is your batteries. The burner fan works on 12v and if your batteries get down the fan speed will drop and then you'll get soot. It can also lead to melting of the aluminium chamber liner which is expensive, but there is a voltage checker built into the circuit, so it shouldn't be a problem. Another possibility is if it hasn't had any maintenance for several years, the chamber could get cruddy enough to cause this.
Ask if you get into it and get stuck.
Jim
