Oil fired heater

check the tank for jelled fuel, add/run kero if needed

put a new filter in the canister or add a filter if you don't have one

pull the gun and take the nozzle out, check the screen for debris, if clean put a new nozzle in same size and pattern, if full of crud see item 1 then run pump for a while without the nozzle to clear the crud out

check air shutter to make sure the kids didn't adjust it on you or that the cobwebs aren't blocking it, get the birds nests out of the chimney and even clean it if needed

if that doesn't do it, then check the pump pressure with a gauge, those Sundstrand pumps last a while but don't like to be run downstream from a plugged oil filter

IMHO

They were really, really ugly. :>)

Allan

(SOME furnace burners in these later years operate at 150 PSI.)

Don't mess with it unless you have a gauge to connect to the pump, and verify the setting.

If it is lighting "hard", and smoking when it finally lights off, the ignition electrodes (some are like a spark plug) may be burned back, so the gap is too wide.

Or, is may simply need a new nozzle.