Burner won't stop burning

We left a job an hour and a half ago, needed the heat for some concrete. Turned off the burner, let her cool down, grabbed lunch and went to the next job. The stack is still smoking… Any ideas…

Your burner is dumping more fuel than it is burning. Either the air/ fuel mix is off or your coil is starting to stop up, therefore not letting enough air flow to completely burn the fuel.

Thank you, sorry don’t get on all that much. Is there a quick fix or do I need to go to a shop? Appreciate your help