Whiff of unburnt heating oil from CH (balanced) flue

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember jim saying something like:

All of those.

Or air setting. Make sure the intake is clear, all the way from the wall to the fire chamber. To set it up properly, a decent flue gas analyser is necessary, but you can get to within 10% of optimum by eye. If it's just an occasional whiff, it might be transient conditions, such as a gust against the flue. What kind of flue? If it's one of those poke-out-of-the-wall stubby types, those are sometimes fairly s*it, it must be said.

When was it properly serviced? There is often a pulse of smell at the moment of ignition but a consistent smell suggests the nozzle/oil pressure/air supply require checking.