Help with smoke/smell on start up.

I have a burner with a Beckett gun that's puzzling me with start up smoke/oder.



I don't believe it's a heat exchanger crack because the smoke is clearly coming out of the damper when the burner fires up.



I wasn't having any problems (that was aware of) but in December I changed the filters and the nozzle and after that I started noticing some smoke on start up. I thought maybe it was a bad nozzle, so I changed it again but it didn't help.



After that I pulled the filters and made sure there was a good seal, not sucking air.



I cleaned the chamber, exchanger tubes, pipes and the bottom of the chimney, mostly a waste of time as they weren't bad at all.



I checked the draft with a manometer and it seems fine while it's running.



I checked all my fittings, changed the electrodes, screen and even swapped in a new coil I had on the shelf.



Some years back I had a dribbling problem that was causing me to melt blast tubes. I installed a Tiger loop at the advice of people here and that solved that problem, so I don't think it's leaking into the chamber.



I did notice how long the motor/pump seemed to be spinning after the flame shut off, and started to think maybe I was pumping oil into the chamber. I installed a fuel stop valve , with a start up delay and that didn't change anything either.



Sooooo.... I'm about out of ideas. I was hoping for suggestions on what else I can check.

Help with smoke/smell on start up.