Oil Burning Furnace lights then goes out and produces smoke

I'm having an issue with an oil burning furnace in my greenhouse. It started up about a week ago just fine, but then I think it ran out of oil. I put about 4 gallons of diesel in it (oil guy can't come until next week) and tried to restart and it wouldn't.
I replaced the oil filter and bled the oil supply line three times. Now it starts when I hit the reset button, but goes off very quickly and then blows out some black smoke after it goes out.

I'm thinking it's an air supply issue or an oil supply issue. The cad cell is fine - new and already tested. Thermostat is fine and tested. The pump seems to be working (or at least is running) and the ignitor seems to work as well. Before I pull out and replace the nozzle I'm wondering if the smoke maybe indicates something else, or points to one of these issues in particular?

of note: each time I bled the oil supply it seemed like there was always air in the line. Also this is an industrial furnace so I don't think there is an air filter.

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