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Probably not a plugged orifice or you wouldn't have a flame at all. Most likely the air shutter on the burner tube is plugged with a spider web or the combustion chamber has some foreign material that burst into flame and shot out the chimney. Another possibility is the gas valve but I'd check the easy/cheap/obvious stuff first.
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.