: It's happened to mine a couple times over the years. Replaced the nozzle and the problem was gone. Yes,the velocity of the fuel oil and the impurities in it,wear the nozzle over time. I'm a mechanic,but it's the same principle as an injector nozzle in a gas or diesel engine.

I found good information on oil burners at the library of our local technical school. The books
included information on maintenance, etc. The Time-Life books also give some information. Nozzles do wear due to impurities in the oil.
One problem I had was knowing which nozzle to buy. In looking back through my records and looking at discarded nozzles from previous service calls,
I found that the service techs had replaced the nozzle with whatever they had on the truck. It took me several tries to find the
smallest size nozzzle that would still do the trick. Finding a good oil furnace mechanic is not easy in the south.
Presumably it's easier up north.